I actually am not certain, but if you'd like to hear it from the horse's mouth you can read this post to the gcc mailing list from an Apple employee, and follow the remainder of the thread:

http://gcc.gnu.org/ml/gcc/2008-02/msg00516.html

For more details from other people who were discussing this issue, you can read a later thread that attempted to go into further detail, although I will not claim it will be elucidating:

http://gcc.gnu.org/ml/gcc/2008-09/msg00416.html