I'm thinking with the onions it's because you're growing from seed. (as in true, tiny real seeds vs bulbs). And you started them really early months before planting so the haircut encourages them to grow extra leaves (not just one or two).
Then again it would not be practical to haircut onions from sets in the field (or shallots or garlic for that matter). So who knows what the results would be? I'm curious... try and tell.
