p0f seems to not be able to identify OS X, leaving it as 'unknown'

For example for OS X Intel 10.5:
 UNKNOWN [65535:43:1:64:M1460,N,W3,N,N,T,S,E:P:?:?] (up: 2532 hrs)

OS X Intel 10.5 and 10.4 and PPC 10.4, among others, need to be
identified in /etc/pf.os

Are they too similar to FreeBSD to have a unique fingerprint?

-Lars