Good questions there, Gar.
And maybe some of the techier folks here can determine a few things.
See, I'm not the most observant or diligent DXer, even on my familiar AM dial. Whatever comes in comes in. I seldom listen at an appointed time, or set aside a day-part (like SRS or SSS).
Some of that informality is apt to change now, with a new shack. But anyway, I'm pretty sure that terrain has a lot to do with that daytime-skip. It might not be the biggest factor. But just about every one of the neat Mid-winter Anomaly catches I've sporadically heard generally followed that arc of the Appalachians.
WKCY 1300 from VA was atop it all at 1:30PM back on Long Island. 315 miles or so
Later that session, WBUC 1460 from WV gave a loud ID.
October 1963 WWHY 1470 from WV was the loudest non-local on the dial (louder than WKBW) for :45 minutes, right around SRS, back on Long Island.
WDJO Cincinnati 1480 was a MWA catch here a year or so back -- 460 miles. In a basement.
WCFR from VT came in at 2PM one lightly-snowing day last year on a portable in a supermarket parking lot while the girls shopped inside.
In other words, each of those catches were off regional frequencies.
Now, those were the only ones this casual DXer *noticed*. So I'm also wondering if the daytime-skip peculiarity works its way up the AM frequencies the way E-Skip climbs up the VHF dial as it deepens.
If so, that might explain why 700 was so loud and 1530 wasn't.