So I looked at AD&D treasures including making a list of monsters categorized by treasure type. The big change is that many monsters have multiple treasure types. Including multiple instances of the same treasure type, for example Q (x10) meaning multiply the Q treasure type by ten.
Again the idea is not to figure out why Gygax assigned specific treasure types to individual monster but rather to understand the treasure types themselves.
List of monsters by Treasure Type
I kept the list in a word doc so you can edit it as you see fit.
A is no longer the Man Treasure Type, is not commonly used either being reserved for the Lich, Locathah, Men Bandits, Squid Giant, and Troglodytes
B to F are still pretty much like their OD&D counterpart with the same rough ascending order of value.
G is no longer just for dwarves. Elves are thrown in as well as a bunch of other creatures.
Interestingly the Roc has been removed from I but more creatures were assigned to this treasure type than OD&D. It also was used a lot with creatures with multiple treasure types, probably because of the Gem and Jewelry values.
H is still the hoard treasure for Dragons. Interesting the white dragons don't get this treasure type. Along with the Dragons the Archdevil Geryon and the Guardian Naga have this treasure type.
Now to the new types
J to N are meant to be assigned to individual monsters. When listed as part of a lair it looks to be incidental treasure. This especially clear for the various varieties of Giant Spiders.
O and P are low value coin treasure types, O is copper and silver, while P is silver and electrum
Q is a used a lot and in conjunction with other treasure types, It is the Gem treasure type.
R treasure type is similar to G but without any magic items.
S is the potion treasure type
T is the Scroll treasure type
U is a high value treasure type with Gems, Jewelry and magic items.
The elite monsters that get this are Orcus, Asmodeus, Tiamat the Chromatic Dragon, and interestingly enough Ixitxachitl Guards and Androsphinxs.
V is just magic items
W is similar to G and R but with map instead of magic items. It only used for the Men, Buccaneers.
X is miscellaneous magic items plus a potion, it been assigned to a lot of monster in conjunction with other treasure types.
Y is just gold pieces
Z is similar to H with a smaller number of magic items the monsters that get this are Men Dervishes, Men Nomads, and Will-o-the-wisp. Looks like there is a lot of wealth hidden underneath those camel sacks.