I have had Time Machine set up on my laptop running 10.8.2 since about midway through October. I try to plug my laptop in once a day to get a backup in. I've been noticing that it doesn't seem to be keeping enough backups. I understand it's supposed to keep weekly backups, and a Snow Leopard iMac my family has, with its drive connected all the time, seems to have four backups for every month. However, my laptop now only has 2 backups from November -- it's got 10/29, 11/11, 11/18, and then 12/1. Why is Time Machine deleting too many backups?