Толщина стенок разная, оттого что глину плохо центровали на гончарном круге.

Because Russian is talking about the property of the walls: the thickness of the walls.

• толщина = thickness
• стенок = of the walls

So the grammatical head of the phrase is толщина, and that noun is singular. English can do the same thing: the thickness of the walls. Even though walls is plural, thickness is singular.

In this sentence, Толщина стенок разная means something like:

• The wall thickness is uneven
• The thickness of the walls varies
• The walls are of different thicknesses

Because стенка is the normal word for the side/wall of a vessel or container, while стена is more for the wall of a room, house, or building.

So for pottery, cups, bowls, etc., Russian usually uses:

• стенка сосуда
• стенки чашки
• стенки горшка

That is why стенок here comes from стенка, not стена.

Because толщина usually takes the thing whose thickness is being described in the genitive:

• толщина стенок = thickness of the walls
• толщина льда = thickness of the ice
• толщина слоя = thickness of the layer

Here the base form is стенки. In the genitive plural, it becomes стенок.

So:

• nominative plural: стенки
• genitive plural: стенок

Because разная agrees with толщина, which is:

• feminine
• singular
• nominative

So the adjective must also be feminine singular nominative:

• толщина → feminine singular
• разная → feminine singular

Literally, разная means different, but here the idea is more natural in English as:

• uneven
• not uniform
• varying

So Толщина стенок разная is literally The thickness of the walls is different, but idiomatically it means The wall thickness is uneven.

Yes. Неравномерная would sound more explicitly technical or precise: non-uniform / uneven.

Compare:

• толщина стенок разная = the thickness varies / is different in different places
• толщина стенок неравномерная = the wall thickness is non-uniform

The sentence with разная is perfectly understandable and natural, especially in less formal language.

Оттого что means because, since, or due to the fact that.

In this sentence:

• Толщина стенок разная, оттого что...
• The wall thickness is uneven because...

It introduces the reason for the uneven thickness.

It is similar to:

• потому что = because
• из-за того, что = because of the fact that / due to

Here оттого что is a bit bookish or explanatory in tone, but still normal.

Because оттого что introduces a subordinate clause of reason.

Main clause:

• Толщина стенок разная

Subordinate clause:

• оттого что глину плохо центровали на гончарном круге

Russian normally separates such clauses with a comma, just as English often does before because when the sentence is structured this way.

Because глину is the direct object of центровали.

The verb центровать / центрировать here means to center. You center what? — the clay.

So:

• nominative: глина = clay
• accusative: глину = clay, as the object

That is why the sentence has:

• глину плохо центровали
• they centered the clay badly / the clay was poorly centered

This is a very common Russian pattern: the 3rd person plural past can be used with no explicit subject when the doer is unknown, unimportant, or just not named.

So центровали literally looks like they centered, but in English it often corresponds to:

• the clay was centered
• someone centered the clay
• they centered the clay in an indefinite sense

The sentence does not mean that a specific group of people is important. It just means the action was done.

This is called an indefinite-personal construction.

Russian often prefers an active verb with an indefinite they instead of a passive construction.

So Russian naturally says:

• глину плохо центровали = the clay was poorly centered / they centered the clay badly

A more passive-style version is possible, for example:

• глина была плохо отцентрована

But that sounds more formal, technical, or written. The original sentence is more natural and conversational.

The imperfective fits well because the sentence focuses on how the process was done, not just on the completed result.

• плохо центровали = the centering was done badly
• the bad quality of the action caused the uneven thickness

A perfective form would focus more on completion of a single finished action. The imperfective is very natural here because the speaker is describing a manufacturing/process problem.

It is the prepositional case because на here means on in the sense of location.

• круг = wheel
• на круге = on the wheel

With the adjective:

• гончарный круг = potter’s wheel
• на гончарном круге = on the potter’s wheel

So:

• на
  • prepositional = location
• на круг would mean motion onto the wheel, which is not the meaning here

Literally it means potter’s wheel.

• гончарный = pottery-related / potter’s
• круг = wheel

So the whole phrase is the standard Russian term for the rotating wheel used in pottery.

The word order is natural, but Russian word order is more flexible than English.

The original order:

• Толщина стенок разная, оттого что глину плохо центровали на гончарном круге.

This presents:

1. the result/problem first
2. the reason second

That is very natural in explanation: The wall thickness is uneven because...

You could change the order for emphasis, but the original is a very normal way to say it.

A useful chunk-by-chunk reading is:

• Толщина стенок = the thickness of the walls
• разная = is uneven / varies
• оттого что = because
• глину = the clay
• плохо центровали = was centered badly / they centered badly
• на гончарном круге = on the potter’s wheel

So the full structure is:

• [Result/problem] Толщина стенок разная
• [Reason] оттого что глину плохо центровали на гончарном круге

That is a good way to understand and remember it.