It's not stupid at all, it's simply how it works. Blockchain.info is not a tool that shows how much coin you have in your wallet, because a wallet is a collection of addresses, not just a single address.
my backup wallet.dat is dead ? muss i greate a new backup ?
Bitcoin-qt generates addresses in batches of 100 and stores them in wallet.dat. Every time you create a new receiving-address, it takes an address from this pool. Every time you send a transaction, it uses an address from this pool as change-address. Once the 100 addresses are used up, a new batch will be generated.
This means that as soon as the number of receiving addresses plus the number of sent transactions is more than 100, you need to create a new backup. And again for 200, 300, 400, etc... Of course, backing up more frequently than that is perfectly fine too.