I'd say this way round is more reliable than having total users being the default. There are clearly servers with thousands of idle accounts (likely bot accounts) that are doing nothing (yet).
At least this way, people will know which servers are actually filled with people who are doing things.
Someone is going to be using those bots for some nefarious purpose at some point. Perhaps they wanted to direct people to that instance by using bots to push it to the top of the "most users" list and have a phishing link in there? I'm just not sure.
I wonder if Kbin has 44k vs Lemmy's 37k due to the Lemmy front page loading bug under Hot or Active. It makes browsing the /All option very annoying, and isn't an issue on Kbin (which is why I'm defaulting to Kbin over Lemmy right now).