I spent a while researching trying to figure that out, and the best explanation I could come up with is that the 11 and 13 T gears extend the involutes of their teeth and modify the addendums of the gear such that they contact the other gear at the same distance a corresponding 12 or 14 T gear would. So, basically, they make the teeth of an 11 or 13 T gear longer and modify the profile of the gear to make sure the gear teeth can still mesh properly. But I don’t claim to really understand it, as there’s really very little information on the internet about gear tooth design.
I’m hoping Paul Copioli can jump in here, I’m also interested to learn what’s “really” going on.
EDIT: Other people beat me. And I thought CD wasn’t supposed to be busy this time of year…