Germination Stragglers -- what all causes them?

[Greatgardens]

It seems like every year I get a couple of varieties that are tardy in germination. This year it was Dubok and Kumato (OP). These were planted in exactly the same manner as several other varieties. The others started poking through in 4 days (my usual with bottom heat). The Dubok took over two weeks and finally started coming up. The Kumato 10 days -- not so bad, but compared to their "efficient neighbors" at 4 days, that's still pretty long.

I don't have any reason to suspect that the seeds were old or poorly processed. What goes "wrong?" Genetic? Something else seed-related? Just curious -- my "crop" doesn't hang in the balance.

-GG