Freebsd standards compliance

[Da Rock]

I'm just in the middle of researching the core of Freebsd (for want of a
better term) so I can understand how to fix and create software and
drivers. Can people clarify my understanding here?

According to wikipedia Freebsd is only mostly compliant with POSIX, yet
BSD/OS is fully- why would this be?

>From what I can tell ANSI C is the standard, and POSIX is an
implementation(?) of that standard (threads, i/o, etc)? Which version of
these standards is Freebsd at- c89, c90, c99, POSIX 1(b,c, etc)?

Excuse my ignorance- for reference I am reading the Developers guidebook
and the architecture book atm :)