Similarly, since all your tomato seedlings are from your own seed (and not hand pollinated, I assume), maybe those 6 weird seedlings have some wonky genetics that just didn't match up well. It's a thing that is not uncommon when hybridizers try to cross different species, like currant tomatoes (Solanum pimpenellifolium) and regular tomatoes (Solanum lycopersicum). Seeds can have different genetics even if they are from the same tomato fruit.
