Not at all, you're thinking of key pools. Here's the thing. Your wallet file generates keys when you need them, but it can't reverse engineer them or figure out if they belonged to it without keeping a list, you see what I mean?
Normally, this would mean you would have to backup your wallet.dat file every single time a new key is generated.
To fix this obvious problem, Bitcoin clients have "key pools". Your client has already generated 100 keys for you, and every time you generate a new key it's just taking one from the list and generating another for that list. There is always a buffer of 100 keys, which means you only have to back up every 100 keys, instead of every key.