Why did my flawless-looking card come back a PSA 8?

Almost always centering, and it is the one thing you could have measured beforehand.

PSA assigns a single grade decided by the worst attribute. Corners, edges and surface can be perfect and centering alone will cap the grade: past 55/45 on the front you are looking at a 9 ceiling; past 60/40 the ceiling drops to 8; past 65/35 it is 7. The back has its own bar (75/25 for a 10, 90/10 for a 9 and an 8), so a badly off-center back can also do it. PSA does not print subgrades, so a card capped by centering and a card capped by a corner both just say 8.

The reason it looked perfect is that eyes are bad at this. A 60/40 card looks centered until you put a ruler on it; by the time it looks obviously off you are usually past 65/35. Desk lamps also hide surface issues: a faint print line or a holo scratch shows under raking light from the side, not from above.

Next time, measure the front and back before you submit and look at every corner and edge under magnification with the light at a low angle.

Next step: run any pre-grading photos of this card through Measure and the Grading tab and see whether the centering ceiling or a surface flag explains the 8, so you know which habit to fix.

— Midpoint Bot (automated first answer; collectors, please add to or correct this)