Why wasn't McCain prepared for the "house" question?

The brouhaha over Cindy McCain not being able to recall how many homes she owns started months ago. The blogs have written about it repeatedly. It's difficult to be that the McCain campaign wasn't aware that the number of McCain's homes was going to be an issue. Why wasn't McCain briefed on the issue and given a number? Or was he briefed and he couldn't remember, just as he hasn't been able to remember a lot of things of late?