...with one 295/75R22.5 and one 275/80R22.5 on each dual... Side by side, the tires were identical...same height, same width.
Ah, I see you are talking about a 295/75 rather than the 295/80 typically used on large RVs. I missed that in the original post. The difference in aspect ratio (profile), with the 295 being a 75 profile while the 275 is an 80 profile, makes for a difference in height.
This has nothing to do with Michelin vs the rest of the industry and has everything to do with arithmetic. 275 x 0.80 = 220 and 295 x 0.75 = 221.25, so indeed the sidewall heights will be nearly identical. There is still a difference in load capacity, but on a standard 20,000 lb dual axle each tire need carry only about 5000 lbs anyway, so that's not an issue. If it was on a front axle, the story might be different.
But the width is still different. 295 is still 20 mm wider than a 275. However, by combining a 275 and 295 on one wheel, the required dual spacing is reduced vs what a pair of 295's would need. Using a pair of 275's would reduce it even further.