Its not an expliot...
I figured this out on my own a long time ago, the Fire Festival food isn't the same as regular food is. The buff they gave is instant, and is not considered "Well Fed". There's a few other items and such you can stack like this.
Although it is just using common sense, this will come in handy for a lot of people that cannot really be arsed to do the quests to collect the flowers in order to buy these similar food buffs. I personally find it extremely useful as I play a Rogue which obviously requires a lot of Hit Rating, especially in the end-game content.
The only comment I have is that there is an ad to "Help feed the hungry children in Africa!" on top of this thread named "Stacking Well Fed".
More of a tip than an exploit id say...
Official Markons hater