Xbox Live Forums Support Rep Confirming Issue
Apparently one of the temp fixes is:
Tremendous news guys! Our LIVE Engineers were able to identify the root of the matter and have run a fix that should resolve the issue. To force your profile to recalculate your tenure (to the correct number!), can we have you delete and re-download your profile to your Xbox 360 console? For a step by step on how to delete and re-download your profile check out our Support article - Download, move, or delete your Xbox LIVE profile!
*Note that when you delete your profile make sure to select Delete Profile Only. This deletes the profile but leaves saved games and achievements. We wouldn’t want you to lose any of the good stuff!
If you do continue to see the incorrect tenure after re-downloading your profile please let us know by creating your own thread in this forum or replying to your thread if you have one already open. We appreciate the patience you guys have had throughout the issue! Really helps us keep our eye on the prize.