Sure, the two most probable explanations seem to be:

• The people in the high group were themselves considered more desirable, and so could afford to be more choosy.
• The people in the high group tended to rate others as less attractive, corresponding to somebody needing to be at a higher percentile in order for people in the high group to be attracted to them.

These two things may seem to be completely orthogonal, but they're not... Giving low ratings to other people's attractiveness was correlated with being attractive oneself. I suspect that there's an element of acclimation /hedonic treadmill – when somebody has the option of dating many people, the less attractive of them start to seem less attractive than they would be if one didn't have so many options.

And the low group's apparent substantially greater responsiveness to sincerity and intelligence once one passes the 50th percentile hints that the difference between the two groups isn't cleanly captured by "desirable people who can be choosy vs. undesirable people who have to settle," though there does appear to be a component of that, and there are also statistical subtleties around the question.