I think it's the carb refeed effect. Chocotobo has a really good description of it. Basically something like normally your carbs are pretty low so insulin and leptin also get really low. The extra carbs in a refeed jack up the insulin and when insulin gets jacked up, so does leptin. Then you go back to your regular lower-carb eating and the leptin jolt gives your system a nice kick toward weight loss. Something like that. I probably got it wrong. If you can find the thread in the search, he explains it in like his 2nd post in the thread. But I've noticed the same thing. I went to a party and ate some chocolate and some sushi and I was like what? I lost weight? And this weekend I purposefully tried a carb refeed and today the tape measure was a little below 30" for the first time.
Female, 5'3", 49, Starting weight: 163lbs. Current weight: 135 (more or less).
I can squat 180lbs, press 72.5lbs and deadlift 185lbs