Every time you change the passphrase you will get a different hash even if you re-use the same passphrase.
This is because of how bitcoin core encrypts the wallet file (it actually encrypts the sensitive data like private keys). You can read more about it here:
https://github.com/bitcoin/bitcoin/blob/6b8a5ab622e5c9386c872036646bf94da983b190/doc/READMEI believe the bolded parts on the quote below may answer your question:
The keys are encrypted with a master key
which is entirely random. This master key is then encrypted with
AES-256-CBC with a key derived from the passphrase using SHA512 and
OpenSSL's EVP_BytesToKey and a dynamic number of rounds determined by
the speed of the machine which does the initial encryption (and is
updated based on the speed of a computer which does a subsequent
passphrase change).