Why would I suddenly lose a bunch of weight, but only temporarily?

It doesn’t happen a lot, but every now and then, I will step on the scale and find I am 8–10 pounds less than I expect. I won’t be dieting or consciously changing anything, and in both of the most recent cases, I didn’t feel especially thinner, no one has commented, etc.

Basically, I don’t believe the scale, even when I reweigh. Then, a week or two later, I’ll step on the scale again and find that the missing pounds have returned, again, with little fanfare or anything being noticeably different.

I tried Googling this and couldn’t find what I was looking for.. They talk about either rapid loss or gain, but not both. Does anyone know why this would happen?