Lose weight after eating a hamburger??

I'm 46 years old, 5'2" at 125 pounds, eat about 1200-1400 cals with typical macros, and don't exercise hardly at all. Since being on MFP, I've tried to find trends to help in what really works for weight loss/gain. The only one I've found so far is when I eat a single Culver's hamburger (and small fries), I lose weight the next day. I understand about fluctuating, but this is a whole pound and consistent each time--once a month or every other week. Do any of you have experience or advice on this? Thanks!