There's a certain bit of allowance they make for sizes outside the "standard size" In the beginning it was a pretty large allowance 1/32 of an inch? I don't recall exactly. But I think that how common very slight trims became has made them all tighten it up.
SGC also rejects for minimum size, but not entirely consistently. For example T206s can be narrow but not short. That's probably to allow for the ABs but gets applied to other backs too. I had one rejected for min size that was as short as another that graded 40 was narrow.
They also reject for odd factory cuts, another I had was unslabbed with the note "miscut top and bottom" Factory but very rough cuts.
And I believe the reason is exactly as DezHood said. Even if the card is fine but factory undersize, many if not most collectors will call it out as trimmed if it's unusually undersize. And while they should be entirely about the technical stuff the company reputation is all about the perception of them by potential customers.
Steve B
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