You both fell for the marketing "spin" (ok its effectively a lie). United and other airlines (plus unfortunately seat guru, which has just followed them, w/o checking the accuracy of the claims) just measure the seat pan (the area between the arm rests). Boeing, because its narrow-body A/C are narrower (the 737 is 7" narrower than the 319/320/321) to try to trick the width ratings, has gone with narrower arm rests as well as trying to jam the seat against the wall. The result is a number that looks better but does not reflect the shoulder width of the seat, which is what really matters (its your arms/shoulders that bump people, not your butt). With the 10x 777, Boeing went with non-existant arm rests, and you can feel the lack of space, not only is the seat pan narrower, but more importantly the shoulder/arm space is gone.

The figures I gave reflect the reality of the seat. It is simply the plane floor width - aisle width/# seats. IMHO it is the most accurate way to look at the issue.

Currently most airlines have a combo of Boeing and Airbus A/C, and Airbus A/C had not always won (e.g. the 777 at 9x has wider seats than the A330/340) and as such they have been careful about marketing it. However, with one airline (Delta) going wider and another two (AA and UA) going narrower, as well as the better availability of data on comfort, these issues are going to get marketed/scored more accurately over time.

But the reason why the 777 on UA is a "torture tube" is not that the seat pan is slightly narrower, it is that the seat is narrower in the seat pan, and the arm rests, and the aisles.