I just wiped the account history and ran a resync. So should be good now. Maybe someone else was using that username in the past.
EDIT: Actually I looked further down the list and I see what you're talking about. Bunch of 360 games marked completed that shouldn't be... looking into.
Ok it's fixed now. Bit of a strange issue even the Xbox app on PC shows those achievements as earned - on the activity overview, the total is correctly shown as 0, but when I click on the actual game (NHL 13 for example) then it shows all the achievements as earned. Added a few extra checks for this so we can avoid adding those though.