Näiden lukemien ero on pieni.

Because näiden lukemien is a noun phrase meaning of these readings.

• näiden = genitive plural of nämä / tämä → these / of these
• lukemien = genitive plural of lukema → readings / of readings

In Finnish, a structure like X:n Y often means Y of X or X’s Y.

So:

• näiden lukemien ero
  literally: the difference of these readings
  natural English: the difference between these readings

This is a very common Finnish way to build noun phrases.

The basic dictionary form is tämä (this).

Its plural nominative is nämä (these), and its genitive plural is näiden.

A few useful forms:

• tämä = this
• nämä = these
• näiden = of these

So in the sentence, näiden means of these and modifies lukemien.

The basic form here is lukema, not luku.

That matters because the two words are related but not identical:

• luku can mean number, figure, chapter, or reading in some contexts
• lukema often means a reading/value shown by a meter, instrument, or measurement

So lukemien suggests measured or observed values, readings, or figures.

Its forms here are:

• lukema = reading, measured value
• lukemat = readings
• lukemien = of the readings

So the sentence is specifically about the difference between these readings/values.

Because the sentence is talking about one difference.

Even if there are multiple readings, Finnish can still use singular ero if the idea is the difference among/between them as one overall thing.

So:

• lukemien = plural, because there is more than one reading
• ero = singular, because the difference is treated as one concept

This is natural in English too:

• The readings are many
• The difference is small

Because it is a predicate adjective with on.

In Näiden lukemien ero on pieni:

• ero is the subject
• on = is
• pieni describes the subject

Since ero is singular, pieni is also singular.

So the structure is:

• ero on pieni = the difference is small

If the subject were plural, the adjective would also be plural:

• Erot ovat pieniä = The differences are small

Finnish often does not need a direct equivalent of English between in this kind of structure.

Instead of saying literally the difference between these readings, Finnish can say:

• näiden lukemien ero
  literally: the difference of these readings

This is idiomatic Finnish.

A more explicit alternative is:

• Ero näiden lukemien välillä on pieni.
  = The difference between these readings is small.

Another common version is:

• Näiden lukemien välinen ero on pieni.
  = The difference between these readings is small.

So the original sentence is perfectly normal; it is just more compact.

It modifies ero.

The core sentence is:

• ero on pieni = the difference is small

Then näiden lukemien tells you which difference:

• näiden lukemien ero = the difference of/between these readings

So the full sentence is built like this:

• näiden lukemien = of these readings
• ero = difference
• on pieni = is small

Not necessarily.

The plural lukemien just means more than one reading. It could be:

• two readings
• several readings
• a set of measurement values

In practice, difference often makes people think of two values, but the grammar itself does not force that interpretation.

If there are several readings, the sentence can mean that the overall difference among them is small.

Yes, in some contexts, but the meaning may shift.

• lukujen comes from luku
• lukemien comes from lukema

Compare:

• lukujen ero = difference between numbers/figures
• lukemien ero = difference between readings/measurement values

So if you are talking about measurements, meter readings, test values, or observed results, lukemien is often the better choice.

The word order here is very natural and neutral:

• Näiden lukemien ero on pieni.

The whole subject comes first:

• Näiden lukemien ero = the difference between these readings

Then the verb and predicate:

• on pieni = is small

You can change the word order in Finnish for emphasis, but the neutral version is the one given.

For example:

• Pieni on näiden lukemien ero.

This is possible, but it sounds more marked or literary. A learner should usually stick to:

• Näiden lukemien ero on pieni.

Finnish has no articles like English a or the.

So ero can mean:

• a difference
• the difference

Which one is meant depends on context.

In this sentence, English usually prefers:

• The difference between these readings is small.

But Finnish simply says:

• Näiden lukemien ero on pieni.

with no article at all.

Grammatically, it is closer to the readings’ difference or the difference of these readings, but idiomatically the best English translation is usually:

• the difference between these readings

That is a good example of how you should not translate Finnish word-for-word too rigidly. The structure is possessive/genitive in Finnish, but the most natural English wording often uses between.

No, that would not be the normal way to say this.

• näissä lukemissa means in these readings
• that does not naturally express between these readings

So for this meaning, use one of these:

• Näiden lukemien ero on pieni.
• Ero näiden lukemien välillä on pieni.
• Näiden lukemien välinen ero on pieni.

Those all express the idea correctly.