Bottom paint bleed into gel coat?

[TheSeaPenguin]

I have noticed on quite a few boats that there is (what appears to be) bottom paint seeping into the gell coat just above the start of the bottom paint.

Wish I could find a picture of it - but my question is what causes this on some boats but not others? Or, alternatively, how do you keep it from happening? Is it actually bottom paint seeping up? Or something else?

Thanks!