Not all activities give the same level boost for a reward, and all activities have a range at which those rewards will drop. For your light, the activity to give you the weapons either drops things at a minimum 6 light above you, or it drops something like 4-6 above you and you've gotten lucky. Armor on the other hand is probably dropping from an activity in that 4-6 range and you're just unlucky. Or that activity only drops up to 5 light above you. Either way, just unfortunate luck, not a busted mechanic