I would assume they usually notice the ink is running low and discard the cards with insufficient ink, and every now and then one slips through (so indeed human error) that’s grey instead of black for it’s ‘K’-layer EDIT: top black layer.
Here are two grey misprints I have myself from other sets than the ones already mentioned:
Also, I was unable to find the specific videos, but I’m pretty sure I’ve seen the Skyridge cards with grey set symbol a few times before in videos of both @TCAGaming and @gemmintpokemon. Maybe they can shed some light whether the entire card is lower on black ink, or just the set symbol in their copies.
I would assume the entire card indeed, and it’s just most noticeable on the set symbol being a larger grey area, as @pfm suggested. But in the current pictures shared by @octaane, the text doesn’t look too much lighter. But a side-by-side would indeed make this a lot clearer. 
Greetz,
Quuador