I've noticed those same variations. For example, my weigh ins for the blog are on Sunday mornings. Out of curiosity I got on the scale on Saturday rather than Sunday last week. If I'd gone with that number I would have lost 7 pounds. But since I do the official weight every Sunday, I only got to log 3 pounds. The variations can be so major but are completely natural. I've learned to not get so discouraged by them.