1) There are plenty of cards out there missing parts with number grades (none that I've seen missing anywhere close to as much as yours). 2 highlighted in this thread already - anyone else have any handy?
2) Grading company guidelines have been a moving target for some time now with some very seemingly random decisions - one of my favorites - cards with paper loss get number grades (I've seen them up to 5's - take a look at T205's), but cards with staple holes (even though there is arguably a greater % of card there than some with paper loss) get automatic 1's
3) Specific to your issue - SGC sent back a sticker on which was printed "TRM" - presume that means Trimmed? Why not "A"? Or nothing? You raised a fair question about what exactly does "TRM" mean?
4) Problem specific to SGC - They DON'T differentiate between "Authentic" - ie. a card that has not been doctored, but for a defined reason does not qualify for a numeric grade and "Authentic Altered" - a card that appears to have been "doctored" in some way. Everything just gets an "A" - VERY confusing