you are not right, difference is much much more than 10-15%, at least 500%, just not adjusting config, run more times same as single. so forgot about # cpu, put nproclimit 20+ everywhere
but this is not bug, it is pool feature to 'help poor miners' boosting their hash power arificialy (hash2)
Ah, now I see what you mean. So the actual "exploit" is not running multiple wallets, but increasing nproclimit far beyond your CPU core number. This is what tricks the pool into thinking that these are all small CPUs and pushes their HPS2.
The multiple wallets are only needed if you have a lot of CPU cores on one machine , because the nproclimit is capped at 40something.
I did some testing on pool.biblepay and this is definitely a thing. The same machines create much more "shares" and a higher "HPS2" (with HPS staying the same) by just increasing nproclimit to 16 or 24 (on 2 core systems).
That being said I think this should be either made clear for all users, or prohibited in some way, because you basically steal payout shares from people with a "conservative" setting ...