It is a crutch for timid teachers...
Some teachers may feel compelled to try to teach you every version of a sign that they know. Why? They are afraid you will go out into the real world, not recognize a version, and think less of your teacher.
The fact is it is quite possibly better to teach a student "one" version and tell students "you will see other versions" -- and then move on to your next concept.
Each weird little version has an opportunity cost.
That "cost" is the taking away of time from some "other" more common sign.
You can either teach four versions of "25" or you could teach four completely different signs in the same span of time. Imagine two students at the end of two different classes:
* one student knows 400 signs
* one student knows 4 ways to sign 100 signs
I'd prefer chatting with the first student every time. They will pick up the variations via self-study and real world interactions.
Notes: