There are quite a few dumb DnD/5e rules from my perspective as a non-DnD player, so it being a rule in 5e is no argument for me. It's also very believable for me that a big area of effect spell applies different damage to the targets, as the caster might not be able to guarantee the same intensity of magic everywhere, or some targets duck and cover or have different clothes or what else which randomly effects the damage. It is a flying spell which explodes, so it effects the targets from the outer side, it is not as if their substance was magically damaged from inside in always the same amount.
If it is a core spell (my party has it on Gale and Wyll), it especially has to be balanced and comprehensible. Of course they can make what they want of it, anything is explicable, and maybe it is a bug in the current state. But I find the current state quite logical and ok.
