Niche directory site. Most directory pages rank as basic, some as intermediate. Pages displaying current data (update hourly) are intermediate.
Search terms I'm always #1 for the site disappears using the advanced filter.
These are not articles, and by their nature not structured for grammar, but contain plenty of text. Looking at the other sites that hold on for the advanced filter, it doesn't look like grammar is their strength either. Detailed complex technical terms (and plenty of them) are driving it here.
Note that it's called Reading Level, not grammar level.