If it works--it probably won't work day after day. Probably your glycogen levels are little higher today, and they'll be a little higher the next. Also, depending on how strict your diet has been, for how long--levels of certain enzymes needed to break down glucose for energy may be decreased, so that you're still burning largely fat even though you've taken in some carbohydrate, keep it up and you have the stimulus needed to upregulate production/activation of those enzymes.

My advice that you didn't ask for is not to look for excuses to eat pizza. Never ended well for me.