My Xbox Live tenure for the gamertag Mattyovers has gone back from '6' to '5'.
I already contacted customer support and they said they can see it should be six years, not five. I haven't been online in over three weeks and just noticed the issue today. My account went from five, to six on the 1st November 2012.
I tried deleting my account, clearing my cache, remove storage devices, inserting a new storage device, powering down the console and unplugging it for 30 seconds, then redownloading the profile. After doing this, it still recovers the account as 5 years tenure.
One of your support teams told me to post a topic right here, as they could see that it should be six years, not five and you were able to fix it for me.