I too am guilty of under grading this coin. I own these from time to time and I have the same problem as others do with these. They always look worse than the grade on the holder which has allowed me to make an occasional great deal on a Type 1. Type 2 grading is a bit more strict than Type 1, and Type 3 being the strictest of all. These coins were well circulated within the banking system and their age reflects their rarity in higher grades, with exception of a few shipwreck coins which are worth a lot less due to their unusual preservation.
I had this 1861 double eagle slabbed about twenty years ago by NGC. The grade is AU-58, which is right on the money. Does anyone think that the previous coin is only three points lower in grade than this one? NO! Here you see another example of "grade-flation."
This one is “only” 3 points better (it is the same type so I’m not comparing apples to oranges). https://www.davidlawrence.com/rare-coin/1713946
In a word, no. If they are “baggy, lackluster and barely even exit at the higher levels,” then there are fewer of them in the higher grades. You don’t drop to standards to get more higher grades. This is even more true for the Type 2s because there have not been any seawater recovery recoveries as there have been for the 1857-S $20 gold. MS-63 and better Type 2 $20 gold coins are scarce to rare.
I am coming in really late in the game and have not researched this, but would systemic low striking pressure and/or worn dies explain the flat details on the seamingly over-graded Type 1 coins?
That might be true for the New Orleans Mint coins. I have never collected or studied those pieces. It's not true for the Philadelphia and San Francisco Mint coins I've seen. These coins were not made in "the dark ages." The mints needed to do their work well because that was one way to beat the counterfeiters. Here is a typical SS Central America 1857-S double eagle. It's not poorly struck.