Probably because OKW start off with a SP that is a tad more expensive than a rear echelon, iirc this is what makes the MP difference at the start.
Fuel is the bigger concern. OKW starting at 5, USF at 15 and Brits at 20. MP is more easily explained, as seen above ^
There's a good reason for this, USF by design lack elite infantry that Obers/Pgrens/Penals provide. They also lack a sniper to deal with those non-doc terminators. Hence their vet 3 is a little better by the time these units are on the field.
2x upgrades if you look at the stats are not as simple as you make them out, compare the MG42 or MG34 to the current tommy bren and you will see a big difference in DPS in favour of the german guns.
That'd be fine, if the elites came out to contest their vet while they're still obtaining it. By the time obers actually hit the field rifles are mainly vet 2 sometimes vet 3 and racks are already unlocked and squads are being given BARs. Making vet 0 obers vs upgraded infantry a larger struggle than need be. Yes the LMG42/34 are better than the allied counterparts, hence why they can only get 1.
If I had things my way they would have kept some form of AT upgrade and their old RA vet, but we were dumped with StG's as a kneejerk reaction and then the flame nade because at the time flak halftrack did nothing vs garrisons, doctrinal sturm flamers did not exist and ISG smoke wasn't a thing. (the ISG was also pretty overpriced back in the day).
But now OKW have been given tool after tool while allied early game has seen a steady stream of "adjustments" (Rear echelon nerf, recent WASP nerf, penal nerf, royal engineer nerf I could go on). But here we are and OKW have these tools, that's fine but they should pay for at least one of them.
OKW wouldn't need these tools if every faction had nondoctrinal tools, specifically flamethrowers. Instead we have lava grenade, and up until a few days ago, no indirect to clear garrisons well. (You had smoke yes, but dropping a flame nade in a garrison doesn't help too much when a 2x BAR rifle comes out and is ontop of you.)