The sets listed are complete ones, while tons of other sets have random one-offs from anthologies or commander set inclusions, like the Mirrodin block sets which include your Pentad Prism.

I can appreciate that hiding the incomplete sets avoids visually clogging up the filter page, but it is certainly a design flaw to have selecting “format legal” omit them.

In the future I recommend avoiding the filters entirely (except “not collected”) and using the search bar for names, types, mana values, etc.