I think there is something to the theory that, in DC's mind, '70 and '71 count as sets from that era. Since we only ever (rarely) go back to '64, '64 through '73 are really the first decade of 10 @ 10s. Personally, I prefer '70 thru '75 to the sixties sets, but that was my sweet spot in music development (I turned 16 in '75).
Musically I would also put '70 & '71 in with the sixties. (To put it
more accurately, I think '67 through '71 is a distinct period.) A
little bit of 1972 might also fit in there, but by then things were
starting to change.
I had one of those "gee, I'm getting old" experiences a few years ago
when I was in a record store and discovered The Supremes, The Monkees,
and The Mama & The Papas were all in a section explicitly labeled
"oldies." That used to be for '50s/doo-wop. Anyway, that mind set is
probably why we're not seeing as many sixties sets -- to the younger
listener that they're trying to attract, it's "oldies" music.