I think I found this one.
Slavegen.gd, lines 63-72 & 187-190, it looks like the stat range for new slaves lets Commoners exceed their caps. Lines 63-72 give the base range a maximum of 65, lines 187-190 gives a commoner up to +15 for 80 total (or 85 for Charm). The same thing happens with Rich slaves and Confidence, with a possible total of up to 90.
Still exists as of v0.5.3.1d