It often means that one could have derived the correct password from the set of rules - but those rules are not shown when asking for the old password

i have had this happen on some websites occasionally while using my password manager.

Sometimes it means the page checking the password is following a different ruleset eg. the main page is case sensitive and the change password page isn't. Sometimes it's stuff like the entered password is silently truncated to a fixed number of characters and because of that won't let you log in. Sometimes it's wierd character expansions being passed directly to the password checking routine (& or similar).