Am I doing something wrong? Note these are not two separately created wallets - I have those.. this is the same wallet, just a backup of a day earlier.
The Bitcoin terminology is the problem. They're not wallets, they're keyrings with a lot of keys. Initially your keyring gets populated with 100 keys, all able to unlock 100 vaults. Whenever you use one key, another one gets added (and none removed) from your keyring.
Thus even your oldest backup contains the keys to the next 100 transactions (=vault contents) you do. That the client can become confused when you start switching keyrings back and forth is not a problem - just run -rescan.