I was just asking because on BAN you can use port 3334 to lower your difficulty. IDK what that means.
If your miner is slow, it can't submit shares fast enough during the time when the block is being solved so you do not get credited once it's solved, with a lower difficulty your shares are worth less but at least you'd be getting fractional reward compared to nothing. If your miner is fast on the other hand low difficulty will cause it to spam shares and a lot of them can be lost/stale and you're causing unnecessary load on the pool so a higher stratum difficulty solves that problem.
It has nothing to do with worker usernames.
Note there's a difference between stratum difficulty and the actual bitcoin difficulty, the stratum difficulty basically sets the benchmark how much work your miner needs to do in order to get a share credited.