Grading gold has always been a bit tricky for me, even in hand as the softness of the metal tends to get me sometimes. It's a whole different world in seeing the effects of circulation as opposed to silver or copper.
That grade seems a little generous. There must be more luster on the piece than is showing in the photo. Still it seems to have more wear than an AU-53 should have.
I view most of these GTG exercises as primarily an opportunity to learn something about the coin series and to hone my own grading skills. In this case, I don't think I've learned anything more than the old "What were they (TPG) thinking when they graded this coin?". The coin has what is clearly high VF/low XF wear, unless it's not wear but rather weak strike. Is the 1874 $3 weakly struck for that year? I pulled up some Heritage sales in AU-50 to 53 and could find no weak strikes. Was there a unique die problem with that particular die marriage? To @johnmilton's comment about the appearance of a bulge on the reverse, a die bulge can consume metal that would otherwise go to filling the devices, thereby leading to a weak strike. But weak strike is supposed to be a negative when grading because it reduces eye appeal. So even, a weak strike does not explain the grade. And if a weak strike is unique to a particular die marriage and explains the anomalous grade, why is the die marriage attribution not listed on the label which would explain to the collecting community why the TPG graded this coin the way it did? I don't have any detailed die marriage resources available to attribute this coin but maybe someone else does? There was commentary about good remaining luster. If what we're seeing is wear, then the claimed remaining luster (I don't see it in the photos) is inconsistent with that degree of wear. If what looks like wear is actually weak strike and thus the remaining luster might well be consistent with the wear, it still doesn't explain why the TPG called this AU-53 since the coin has a huge debit due to lack of strike and thus eye appeal. I don't think luster outweighs that. So, I'm not buying the TPG grade at all at this point. I think it's significantly over-graded even by debased market grading critieria. I suppose all my points are debatable and I'd be happy to hear opposing views.
It's been my experiences that die bulges on gold like that, especially from the Philadelphia Mint, are quite uncommon. That's why I didn't like the piece. I still don't like for the grade. It's missing a lot of design detail, and it's get a lot of rub. In the "raw coin days" this would have been a VF from the legitimate dealers.