Accessing Time Machine without external drive?

On my MacBook Pro (came with Lion, now has Mountain Lion) with an SSD, I keep pretty current with my Time Machine backups. However, when the backup hard drive is not connected, I can still enter Time Machine and browse older states of files and folders. I assume this has something to do with Versions, and it's pretty cool.

However, on my parents' 2008 iMac (HDD, not SSD, if that matters), I put Mountain Lion on it a couple weeks ago, made a couple Time Machine backups to my external drive, and disconnected the drive. But when I try to enter Time Machine on the iMac, it says it needs the Time Machine drive to be connected.

So my question is, why does the MacBook Pro let me enter Time Machine with no external drive but the iMac doesn't? How do I make it go into Time Machine?