When you look at an artist page, what you’re seeing under Main Albums or Singles and EPs are album on which the person is credited as Primary Artist at the album level.
You should be able to see this on the album’s Edit page, on the Edit Album tab:
Any other album on which this person has a performance credit (ie not a Composer or Production credit) will show up under Appearances, like the 4 albums in your screenshot.
We also split albums out of the Appearances section if they a certain number of those albums all have the same Primary Artist.The idea here is that Miles Davis might have appeared on 100 albums in your collection, but there’s real context we can highlight by splitting the 22 Coltrane albums out into a “With John Coltrane” section.
For that to work properly, Miles Davis needs to be credited on all 22 Coltrane albums as a performer (meaning he’s credited for playing trumpet).
So, I’m guessing the reason you’re not seeing every Led Zeppelin album on the Robert Plant page is that, sadly, the metadata we have for some of those albums fail to credit him for guitar or whatever.
Hope that helps!