El porcentaje sube del diez al doce, pero la diferencia exacta no siempre se nota.

Because Spanish normally uses the definite article with nouns much more often than English does.

Here, el porcentaje is the subject of the sentence and refers to the percentage being discussed. In English, we might say percentage goes up in a note or headline, but in normal Spanish, el porcentaje sounds much more natural.

So:

• El porcentaje sube... = normal Spanish
• Porcentaje sube... = sounds incomplete or like headline style

Literally, it means goes up from ten to twelve.

Because the sentence already mentions el porcentaje, the natural interpretation is:

• from 10% to 12%

A very important point for learners:

• from 10% to 12% is an increase of 2 percentage points
• mathematically, it is also a 20% relative increase

In everyday Spanish, del diez al doce usually just describes the movement between the two values, not the technical distinction unless the context makes it important.

Because Spanish often treats the numbers here as nouns: el diez, el doce.

That gives:

• de + el = del
• a + el = al

So:

• del diez al doce
• literally: from the ten to the twelve

This is very common when talking about:

• percentages
• scores
• levels
• dates or ranges in some contexts

You may also hear de 10 a 12 in other situations, but del 10 al 12 is extremely common when the numbers are understood as specific points on a scale.

Because subir can work as an intransitive verb meaning to rise, to go up, or to increase.

So:

• El porcentaje sube = The percentage rises / goes up

By contrast, subirse usually means something more like:

• to get on something
• to climb up
• to move oneself upward

Examples:

• El precio sube. = The price goes up.
• Me subo al autobús. = I get on the bus.

So in your sentence, plain sube is the correct choice.

Yes. El porcentaje aumenta... would also be correct.

The difference is mostly one of style:

• sube = very common, natural, slightly more everyday
• aumenta = a bit more formal or technical

So both work:

• El porcentaje sube del diez al doce.
• El porcentaje aumenta del diez al doce.

In many contexts, sube sounds a little more conversational.

Because once el porcentaje has already been mentioned, the unit is understood.

So Spanish can say:

• El porcentaje sube del diez al doce

instead of the fuller version:

• El porcentaje sube del diez por ciento al doce por ciento

Both are correct. The shorter version sounds more efficient and natural when the meaning is already clear.

This kind of omission is very common in Spanish when repeating the unit would be unnecessary.

Here, se nota means something like:

• is noticeable
• can be noticed
• is perceived

In this sentence, se is not really reflexive in the everyday sense of oneself. It is part of a very common Spanish structure that makes the action feel less personal or more general.

So:

• La diferencia exacta no siempre se nota
  = The exact difference is not always noticeable

You can think of it as similar to:

• se ve = it can be seen / it is visible
• se percibe = it is perceived

The subject is la diferencia exacta, so the verb is singular: se nota.

Because in Spanish, adjectives usually come after the noun.

So the normal order is:

• diferencia exacta
• cifra exacta
• resultado claro

Putting the adjective before the noun is possible in some cases, but it usually sounds more literary, emotional, or emphatic.

So:

• la diferencia exacta = normal, neutral Spanish
• la exacta diferencia = unusual, marked, and not the natural choice here

Because pero introduces a contrast, while sino usually corrects something after a negative statement.

Here the sentence says:

• The percentage goes up from ten to twelve, but the exact difference is not always noticeable.

That is a contrast, so pero is correct.

Use sino in sentences like:

• No sube al doce, sino al once.
  = It doesn’t go up to twelve, but rather to eleven.

So:

• pero = but
• sino = but rather / instead

Because no siempre is the natural way to say not always in Spanish.

So:

• no siempre se nota = it is not always noticeable

By contrast, siempre no se nota is not the normal way to express that idea.

Spanish usually places no before the part being negated, and no siempre is a fixed, very common pattern:

• No siempre entiendo todo.
• No siempre es fácil.
• No siempre se nota.

Because pero is connecting two full clauses, and in Spanish it is standard to put a comma before it in that situation.

Clause 1:

• El porcentaje sube del diez al doce

Clause 2:

• la diferencia exacta no siempre se nota

So the comma helps separate the two ideas clearly:

• ..., pero ...

This is very normal punctuation in Spanish.