I don't have a dog in the calories fight. But I had a similar experience to you. I was following a Paleo diet for about a month before coming here to PB. I started early in July and got down to about 250 lbs one Friday. I weighed myself on Monday and was 256! The weekend was not bad food-wise, I ate mostly primally, but not bad, I hadn't binged. I had exercised, though not as much as the preceding week . I was at a loss to say how my weight could've changed so much.
So I think that during the week I had become dehydrated, I was exercising too much maybe and depleted my Glycogen stores. When I eased off of the throttle over the weekend, the stores went up and I replenished my hydration level.
I don't look fatter, clothes still fit so I'm not too bummed by it, but I have put the scale away and will now only weigh myself 1X/week.
Height: 5' 10"
Starting Weight: 292
Starting Primal Weight: 275
Current weight: 224
Goal weight: 172
Body Fat 30.5