I know it does get frustrating. And yeah the bun and the cookie could have caused some inflammation. Plus there's just the human margin of error when we measure.
I noticed with myself, and I've read in a lot of posts, that many of the changes happen on the inside first. You know, the crap covering your vital organs.
There are a few scales that seem pretty accurate for bodyfat, but not all are, and you pretty much have to use them first thing in the morning only, right after you pee, or the results will be all over the map.