Why are some squash smooth and some lumpy?

Question:From the same yellow squash plant we get some that are smooth and some that have lumps all over. Why?

Answers:
Haven't planted squash in a while but I think the smooth are young, and the lumpy are old. I'm talking about yellow squash. Now, there are other squash that it is their nature to be lumpy. Can't name them right now, but I'm sure it's got to do with the kind you planted!