Board Thread:The Quarters - Q & A/@comment-207.255.197.238-20141203052630/@comment-24150802-20141204091846

Have a look at the Buff page if you'd like more information. In brief, it's likely to be because either one of the people in your party cast a beneficial skill that increases your stats for a day, or because yesterday you finished all your Dailies (a "Perfect Day") which gives you increased stats the next day as an extra reward.