To distill my reason for doing so down to a simple point:
Mathematical comparison should encompass more than DPR, but I don't have the time or inclination to do more maths. This was done a year ago when dealing with an obstinate forum lurker. I'd much rather design fun stuff and play the damn game. Maths just aren't enjoyable for me, so I had to distill the impact of the Champion's 2nd fighting style into a limited formula only accounting for DPR. For example, choosing Protection WOULD have an effect on the Champion's overall combat effectiveness, but would be harder to compare and frankly more maths than I'm interested in. So beginning with A = A; Defense = Defense, and then just adding Duelist to Champion was much simpler.
As to your query about feats, I deliberately left out feats because both Champion and Battlemaster get equal #'s of feats, being fighters.
Also, simplicity of comparison was what I was aiming for here.
Yes, it's easy to criticize "well you didn't account for X, Y, Z." If that's an issue for you, do your own maths and share them.