Hash of backed up wallet file not the same as hash of wallet currently in use A couple of months ago I had backed up the wallet.dat (I use bitcoin-qt) file and saved it some place else.

Lately I've opened the data folder for the current running node (also bitcoin-qt) and I compared the md5 hashes of both files and they are different.

What caused this? Surely my private keys have not changed so why is the wallet.dat file not exactly the same as the backed up version?

Thank you
The wallet file contains the hash (and more) information about the tip of the chain it has been synchronized to. This means that generally when loading a wallet into a running node, it will be modified every time a new block comes in.

Furthermore the wallet file also contains all transactions affecting the wallet balance. If you sent or received any transactions, that too will inevitably modify the wallet file.