I get it. I don’t remember how it was in Foxit because I ditched it for Sumatra long ago, but the choice in Calibre (for EPUBs) is to jump to the next bookmark regardless of the level. For example, in an ebook with several short stories, some of them with chapters inside, you’d jump from story A to story B to story C, then to the second chapter in story C (let’s say the first chapter coincides with the first page of the story), then to the third chapter, then to story D.
Some people would prefer otherwise, sure. I would accept the alternative of jumping only between top-level bookmarks. I’m just saying that having this feature in any form would be better than not having it.
(But, as I said, it’s not a showstopper. I’m extremely happy with Sumatra as it is.)