Btw the “good quality” Beta Playtest cards are not “randomly distributed”, I mean there are 3 submissions with them (or at least I only found them in 3 submissions, there is almost a complete set, with no duplicates).
As the “good quality” are distributed in (at least) 3 different submissions, it can’t be based on the difference in how the image was taken. There is definitively a “good quality” and a “poor quality” version.
Moreover those are the only Beta Playtest cards which show a lot of dust (like black spots, you can see some on “good quality” Ditto and Psyduck) on every cards. So I am sure they are originaly part of the same set.
Yes it could be as simple as a new printer indeed. So this would mean that Alpha and Beta playtest cards were developped in parallel as there are both Alpha and Beta Playtest cards in “good quality” versions, so they would be printed with the same printer (if the new printer thing was the explanation).
EDIT : Another example where the difference is more “obvious”:
And again there is 1 copy in “good quality” and the other ones (I found only 2 of them) are in “poor quality”.
I will not do that with all the Pokémon, so I hope you get my point ^^