let me guess... you are using an intel Mac? Well, i have a MBP and have the same problem. Let's say i want to start a new document that is say 8.5 x11? If i type that in, it changes to 8.4999999 x 11. For some reason, the 11 usually stays, but other numbers change like that too. As far as i know it's a macintel problem.
-JE