Can someone explain why my farms rates plummet when part of the roof is removed?

Recently I built a [general mob farm](https://discord.com/channels/816095921241980968/816353791259181088/1272297929649946708) by Pags5 (discord). It consists of 18 small pods of spawning spaces and a TNT "distribution system" that throws a tnt into each of the pods every few seconds. It's rates are about 220-240k drops per hour. I have already built it in a desert perimeter in my survival world and it's working great.


However, I recently realized, that since I am using the "Husk drop Sand" Vanillatweaks datapack, it would be possible to adjust the farm to allow for husk spawns. Husks need the block they spawn on to have sky access. So to make this happen, I have built a flying-machine-system that removes part of the roof above the pods at night, so during that time, they can produce some amount of sand.


I have now tested this in a creative test world and have noticed an extreme discrepancy between the farm running with the roof open vs closed.

* When the farms roof is closed, I am getting \~225k drops per hour (tho no obviously no sand) * When the farms roof is open, I am getting \~125k drops per hour (including \~9k sand)

I don't really get why that is. After all, the block the mobs spawn on are on y-63, so removing the roof above is reducing the total height map of that column by more than half (3 out of 5 blocks). There is also no light sources blocking any spawning attempts.

The only explanation I can come up with is that the open roof in some way interferes with pack spawning. Unfortunately the [wiki](https://minecraft.wiki/w/Mob_spawning#Pack_spawning) wasn't clear enough about the influence of y-level differences on pack spawning for me to get my answer there. For reference, the highest blocks of the roof-contraption are the redstone-repeater lines at y-56.

Can anyone explain this situation?