Your onions are much more likely to bolt if planted too early. Whereas bunching onions are often pulled up, with one stalk being replanted and, the whole "bunching" thing starts over again.

If I understand the whole "bolting-bunching" thing correctly: When onions bolt, the bulb is actually starting to divide or multiply (bunching). With bunching onions this (bolting) is exactly what we want them to do.