I have a theory..
getting COUNTED and receiving CREDIT for backlinks are 2 different things.
Theory: I think that Google counts all of your backlinks, then subtracts that number from the number of backlinks it gave you credit for. If the remaining number is too big, you may get a penalty.
Just a theory.
It's why we trade links with sites spanning many different ip ranges and class c's @
www.hardlinks.org to gain your backlinks.