Wow, you're onto something here.
I switched up to 'advanced' and the 'blank' site (mentioned elsewhere) ranked #2.
Our English grammar site (grammar and English experts/teachers) is nowhere to be seen.
UGC - in general, is casual grammar. I'm betting somebody did not think this through :(
It would follow, from this initial observation (blank site) that the filter works by finding mistakes (more so than finding correct grammar). Therefore, the less content -> the higher you'll get scored by this.
Is this is being used as a signal?