I feel sorry for you that no one had replied, so i'm gonna join in and try to think together with you, cause i was wondering the same.
Out of pure guessing, i think that when you have a mining farm, you mine a block very fast.
When you are finished with the block, the server needs to affirm it and send you a new job, which makes a small delay. This delay can decrease the profits over time.
Now the next question is, why isn't everyone using the highest difficulty? I'm not so sure about this, but i believe it has something to do with having to finish a block before you log off or your software crashes. I don't know what happens to a block that gets half mined. I think the block has to be mined in a whole for it to be confirmed.
When you mine with low difficulty, the block is split in pieces so everyone has a part they can complete in a block.
Hope this helped. It's probably not the correct answer, but it sounds obvious.
This was my first post btw. lel hah, never sell, HODL!