Why must a complement class be infinite on universal statements? (Popper - The Logic of Scientific Discovery)

I`d value the generous help here of the enlightened reader:

(Karl Popper) Logic of Scientific Discovery > 33 Degrees of Falsifiability compared by means of the subclass relation

Here we have the definition of subclass:

(1) A statement x is said to be ‘falsiﬁable in a higher degree’ or
better testable’ than a statement y, or in symbols: Fsb(x) > Fsb(y), if and
only if the class of potential falsiﬁers of x includes the class of the
potential falsiﬁers of y as a proper subclass.

Fsb(y) is a subclass of Fsb(x).

The following text puzzles me:

If (1) applies, there will always be a non-empty complement class.
In the case of universal statements, this complement class must be
inﬁnite. It is not possible, therefore, for the two (strictly universal)
theories to diﬀer in that one of them forbids a ﬁnite number of single
occurrences permitted by the other.

The first senctece i understood (If (1) applies, there will always be a non-empty complement class.)

My question is:

Why must this complement class be infinite in the case of universal statements?

In the following picture you can see Fsb(x) containing Fsb(y). The non-empty complement class ist limited by the borders of the rectangle Fsb(x). The following sentence completely eludes my undertanding:

It is not possible, therefore, for the two (strictly universal)
theories to diﬀer in that one of them forbids a ﬁnite number of single
occurrences permitted by the other.

Probably Fsb(y) permits a finite number of single occurrences that would be permited by the other here Fsb(x).