Seals are often carved in such numbers by painters that it will be impossible to know them all.
Some painters did also carve the same seal again and again. We might know a few variants,  but do we know all?
Seals and signatures will look different and should be valued, according to the material on which they are placed.

In calligraphy it is a mark of quality and education to write the same character with slight variations. So, of course, also the signature is subject to variations!
Taken from a set by Tani Bunchou, we clearly can see in this example the intended differences between the three signatures.

Signatures will change according to the space available, the style of the calligraphy used for the inscription, even the style of the composition, etc.
Signatures will change over an artists life again and again.

Often the signature and seal prints, are valued far to much.
It would be much better to check carefully on characters not belonging to the signature. The painter and the forger will do care less here and fall easier in to their own writing habits.