Why is there variability between specific fibrous proteins?

My textbook says that globular proteins can have no variability - which I understand because if you took for example amylase, if the amylase molecules were each slightly different then the starch molecule wouldn't fit into the active site.

However, it also says that fibrous proteins such as keratin and collagen can have some variability. Why is this and can you give me some examples?

Good question!

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