Edit: after reading the thread more carefully, I think I may have jumped the gun in announcing mystery solved. But I'll leave my post below, for what it's worth.

Different gearing used on the two bikes. 50T vs 48T big rings is 4% higher gear on the BMC vs the Trek, even if the same tooth cog was used on the cassette. That's a difference some riders will feel, I would anyway. Especially if you're climbing or accelerating.

It's not always possible to get exactly the same gearing. Suppose you're climbing on the Trek in 48x23 or 2.087. Then you switch to the BMC with the 50T big ring. 50x23 is 2.173 and 50x25 is 2.000. That is 4% difference between each of those gear ratios. (This assumes same tire diameter and crank length.)

Which is, IMO, when you get up out of the saddle.