Because it runs the hash again on the new password against the old one, if it matches the old one you are told to change it as you used the old password again.

[–] Mothra@mander.xyz 4 points 1 year ago

Yes yes but I don't mean when I'm told to change one. I mean when I'm trying to login as usual, password doesn't work, so I change it. Just to test of the password I was using was wrong, that's what I use first- and it's rejected.

I remember Epic would do this on a DAILY basis at some point last year. It was so irritating. "Ah yes the brand new password from yesterday that worked yesterday but that we didn't recognise on the login page today? Well we do recognise here on the reset, jokes on you!"