We noticed this also when we were looking to book for DCR this December.
After we'd first looked, and got a price, the in-laws said they would come with us so we had another look, both with them sharing, and the original quote which was for 2 adults and 2 kids.
I was surprised to find that over the space of 1 day the original price we'd got was no longer available. We got booked up pretty quick before it went up any further!!
I just assumed it was to do with the forces of supply & demand!