Allotments 4 All

I live in Belgium and only buy organic eggs which come in boxes of either 6 or 10.  Other eggs on sale in the same supermarket come in boxes of 6 or 12.   

I don't understand why.  I only have one recipe that needs 5 eggs so 10 seems a daft number to me.   Most use 1, 2 or 3 - all factors of 12.