Assume for a moment that there is such a thing as an objectively good album. Assume also that there's some luck involved in actually making one, not just skill. You have thousands of bands churning out albums.
Let's assume also that the critics are capable of recognizing an objectively good album at least better than chance.
Now picture a band making album after album, and they succeed in making an actually good one, that gets noticed by critics and called out as great. What are the odds that their next album will be as good?
Not great! If there's luck at all involved, it's very likely that their first album, which managed to get them on critics radar against thousands of rivals, was an outlier in terms of quality. They'll revert to the mean in album two.
That's enough to explain critics typically rating a second album lower. Even if they have widely varying ideals for what a good album sounds like.
For fans it's another matter. Fandom is a social phenomenon, it's never just about the music. It's also about the role the band plays in your life. Not just things like parasocial relationships, although that too, but think about it: If you find another artist that sounds just like artist you know and love, do you immediately jump ship? Of course not. Not even if it turns out that the other artist was "first" in making this style of music. There is such a thing as niches in music, once one is personally filled for you, say you've got all the happy party dance music you needed this month, you're not going to care if there's some objectively 1% better music out there.
So it's also understandable that fans rate second albums higher than critics. That just means they keep fulfilling the role they won in their listeners lives.