what's up with these

I don't get what's going on with the marigolds. I sowed this tray with over a dozen seeds, neatly spaced, and only four came up. They were started a few weeks later than last year- in my second group. Do marigolds need to be crowded to germinate well? is it like sowing radishes with carrots, one helps the other break the seed coat? A bit puzzled, but I went ahead and sowed more marigolds today, more thickly with plenty of seeds overlapping each other. Hm.

In the tray of 'big' tomatoes, the three from a paper towel I folded in half, all sprouted! So that worked just fine. Of course, I still have to wait and see if they are more or less susceptible to disease (the fermenting method, which I skipped, is supposed to kill some pathogens on the seed). They're the middle row here.