Personally I’d say this is more nuanced.
Edges/corners - 9.5 edges and corners usually means they would cross to PSA 10, especially on newer certs. Op’s is older and I can see whitening on the scans. 9 edges most likely not unless it’s a card from a set that has lot of factory silvering like Spanish base or English Jungle or Japanese Neo 4 which has factory corner issues (of which PSA makes allowances). 9 corners is almost certainly a no.
Centering - 9.5 centering is a definite yes, 9 centering is a maybe if it’s actually a 9 centering. Centering should be consistent but often grading companies love to make this inconsistent.
Surface - depends. Most big psa 10s base 1st Ed holos have some surface defect eg. Almost all PSA 10 shadowless Chanseys have a minor print line (which would BGS 9 the surface), so it really depends on the specific issue on that card and if the overall card has good eye appeal.
Overall I’ve found 9.5 surfaces are 9/10 times basically pristine. A 9 surface typically means a more significant print line, but it also depends. It could be a minor issue. For example, my shadowless 9.5 Charizard has a 9 surface caused by two minor factory print dots (about the diameter of a needle) under the right wing. I’m surprised BGS 9’d the surface to be honest from this, as from their grading scale this should be 9.5 and at PSA this wouldn’t detract from the grade.
Then you have bias - it’s very rare to bump an in-slab card from one company to the other. Most need to be cracked to remove said bias. It’s also tough doing it from the same company, as you say, especially if it’s a very high end card with a low pop.
I’ve only done one Black Label from a Pristine BGS 10 in the slab, and one BGS 9.5 shadowless Raichu that was a BGS 9++.
Shows are often better for in-slab grade reviews but it’s still more tough than cracking as the bias still exists.
I’ve sent another BGS 9++ shadowless Raichu which is the cleanest base card I’ve ever seen (was a PSA 9) about 4 times into BGS for the bump (in the slab). All times denied. I KNOW it deserves the bump, so it’s frustrating. I may crack it next time.
Basically, everything is a mess and grading is a matter of chance(y).
