HD makes note fade-in (transitioning from being 0% opaque to 100% opaque) slightly quicker compared to no-mod, so it gives less reaction points.
Kurtisdede wrote:Why does DTHR plays give more Reaction skill points than HDDTHR ones do?
example: AR11 DTHR plays are worth around 840 points, AR11 HDDTHR plays are worth around 800-810
edit: Also, some FL plays simply aren't counted - maybe it's because they're short? For example,
on here http://osuskills.com/user/Kurtisdede/skill/reaction you can see I have a FL play on "toby fox - Bergentrueckung (Mazziv) [Checkmate]" but, it doesn't appear on http://osuskills.com/user/Kurtisdede/skill/memory
Maps with FL that are short might not give a Memory rating, since the visible area doesn't shrink enough for the map not be sight-readable.