This is how FileVault encryption on the boot volume is set up, whether done at time of install or via System Preferences after. Eg
here.
If you google "FileVault user password" you might decide not to try making it different. While a few people do make them different deliberately, as you are asking, most hits are from people for whom FV and user password have become different and they want to make them the same. Apple expects them to be same, and doing things different from the expected way can make life difficult.
My advice would be to change your user password to a stronger one and keep them the same, which I have just done.
(PS don't confuse the normal user/FV unlock key, which we are talking about here, with the
FV Recovery key which is only used if you forget your log in, unless you opted to allow recovery via Apple ID.)
NB turning on encryption for external drives is different. The unlock key is not linked to user account password, though of course you can choose it to be the same.