Allotments 4 All

I've started to germinate some tomato seeds. I placed 3 seeds to a pot with the idea of thinning to the strongest seedling leaving one seedling to a pot. Out of the three pots, one pot has three strong seedlings and the other two have nothing. Why couldn't one seedling in each pot germinate rather than three in one and nothing in the others? It's a bit like dropping butted toast, it always falls butter side down.

It's that horrible man, Mr. Murphy.