[Nightshifter]

My assumption is simply that a AI plane often is flying as if it is perfectly trimmed, and therefore isn't losing any speed through minor course changes that a human without trim is subject to.

I agree with the above; the AI may be dumb in some respects BUT they simply do not know how to fly the aircraft incorrectly therefore are always perfectly trimmed hence faster than your average human player. We humans have to actually think and remember to trim properly to get the extra speed whereas they just do it instantly and automatically.