Okay, this is a known bug, but I wasn't able to find anything easily on the forums... when sorting in the Sent folder, the To: column doesn't sort properly.
Bug 6830 - sorting Sent folder by To column sorts wrong
This seems to be an old old bug. Apparently, the 'solution' was to disable sorting by the To: column until it could be fixed.
Bug 12734 - Message view + in:sent To column header allows sort (which wasn't done it seems)
Just an FYI to any who may encounter it, since I just had a user ask me today. And if we could get at least 12734 fixed it would be nice. (6830 sounds like it might involve tons of work) Any who need, pls vote.