I have spoken with reps about this data point, and have reached a dead end. It is not organic data, like your star review metrics that go up or down.
From what I finally gathered speaking to the right rep, after a ton of calls, and closed tickets I found the following. I know it doesn't make a bit of sense, in brief, consider it dead data.
It is a data point that is based annually, and capped at 20 when ever the window is open. I have no idea when it actually renews- maybe your yearly anniversary host start date. I noticed mine right off the bat as I was striving to reach the esteemed super-host, and it stopped after 18 reviews. Since two guests didn't leave a review, mines frozen at 18. I'd be interested in hearing from others, who would say otherwise, but I think this is it.
One of these days I'll get around to making a suggestion/feedback telling the team just how pointless and misleading this data point is. How misconstrued could it be looking at someone who has 100+ reviews and only had 19 recent sparkling clean? Yeah, go figure.