I've always considered population to reflect the unit value at later stages of the game, while resources rather take the role of reflecting the value of the unit right out of the gate. Obviously it's not as black and white as that, but the general idea works well.
POP always defined for me how much combat power you could squeeze into your army. Units that scale very well via upgrades and veterancy should therefore be more expensive in population. Buying a unit or upgrade is an upfront investment, for which you usually need quicker return. Goes more towards the "shock value" of the unit.
I agree with Sanders that these values are balanced within their faction, although I'd say "mostly" instead of "only". We've regularly had standardization of units or abilities across factions, and this makes sense to some extend.
Regarding your example: I think in the end probably no one would really notice the difference between a 16 POP or 15 POP Firefly or Jackson. The Jackson should not be overly expensive since it's your only AT unit to really scale into the late game as USF, unless you want to spend all your munis on those special AT rounds of the 57mm ATG. I think this is why it did not get a price increase: To not risk gating USF out of AT options more than necessary. On the other hand it performs so well that Jacksons need to be population intensive. I assume this is why they cost both less resources and more POP than the Firefly.
We didn't forget anything, Rangers were purposefully left at 10 pop because of how effective tripple elite Bazooka Rangers are and we didn't want to encourage spamming them. Thompson Rangers being affected by this was a sacrifice that was made deliberately. If it wasn't for the zooks they could've been 9 pop like other elite infantry.
Quick question:
Was it considered to give Thompsons a tiny buff then to make them on par in value with triple zook Rangers? If so, what was the reason against it?