The OpenSim NPCs do not count into user statistics. At least not into the once i posted, those are from hypegridbusiness, which directly collects them from grids. It's users that logged in with a grid account within a month.
But there is for sure a higher percentage of alts in OpenSim, i have 3 accounts myself on 3 different grids, because you can just copy your shit from one grid to another. But i just logged in with one in the past month, so i guess i count as one.
In SL you have to buy your shit again and invest money to look decent on an alt.
Just to explain that a bit:
If you own a sim you can use the NPC functions to copy a persons appearance. So you can make an NPC that looks exactly like that person and you can make it do whatever you want. And there is no need to keep some viewer running.
But there is also an army of people and old roasties who are fighting against it, because everything that looks better than >10 yo SL content is evil. The main enemy of opensim are the people who currently use opensim.
With all the good stuff that OpenSim offers, it is just totally unusable if you don't bring your own community with you.