I HAD to respond to you because I don't know if I have ever heard of anyone else having that issue! Years ago I was heavier (140 territory) and I would eat properly, exercise a lot, but lose maybe 1 pound. Then, every few months, my weight would drop by 4 pounds, and stay there. It wasn't one of those weight losses that would creep back on, because I was always 4 pounds lighter, until it happened again.
It was the weirdest thing! That still seems to happen to me now as well. I was down to 124 awhile back, then after careless eating and not enough working out I'm back to 129. I'm waiting for that 4 pound drop haha! But, I'm sorry I'm not more helpful, I have no idea why it happens