Not common across the Board but individually some will be 'common'. Examples...
TOS 28 has four copies in 9.4, two in 9.2, and 3 in 9.0 . My 7.0, previously a decent grade, now has 27 copies graded higher.
TOS 38 has 2 copies in 9.6, 3 in 9.4, 3 in 9.2, and 5 in 9.0 . My 8.0, again a decent grade, now has 10 in grade and 22 graded higher.
The high grade books seem to appear in bursts. Been actively collecting them for about 7 years and it's common not to find any for several months - then 5 to 10 all appear in the same auction or in close proximity !
I do agree that I've never seen so many uber high grade slabbed copies for sale than in the last 6-9 months (even the number of listings for mid-grade slabbed copies has greatly increased). Really challenges the checkbook control sometimes but I'm just not willing to spend $7K on a TOS4 or TTA 5 in 9.2.
Watching the census count for the PHM books since 2013. On average, about 60 books were being slabbed a month for the 164 PHM issues. In 2014, that approached 90 a month and once the Guardians of the Galaxy movie came out, every available copy of TTA13 was slabbed and the monthly count regularly exceeded 125 books. Maybe the Groot effect drew more attention from collectors, dealers and flippers to this genre ? Seeing more and more of the other company's slabs appearing as well.
Can't hurt that the prices are now significant & consistent to get sellers motivated to let them go.
-bc