On top of what you've already pointed out (and me seeing the same thing first hand about 6 months ago), I believe there may also be a language string error. In UBBT75x's CP, it should read "Server Time Offeset" (as it already does), and in the My Stuff > Site Preferences (User Preferences) it should read as "Forum Time Offset."
A lot of this has been rewritten for UBBT76x series (not merged to ubbcentral's code as of this post).
I did see exactly what you are pointing out. There definitely is an inconsistency in one location, which conflicts with all the other locations that have constant times.
for reference:
UBBT75x uses server time offsets.
UBBT76x uses timezone offsets (UTC)
