It’s there to keep Items in the same order that they entered, this means merging Items is ONLY allowed if they are next to each other. The Queue Hopper is there for keeping the order of different Items.
The exact Stacksize only applies to actually outputting the Stack.
I think I might also have done something to make sure the Slots only ever contain a divisible of the exact stacksize per Slot, like in your 15 case it would be 60 instead of 64. But i might be wrong about that.