What size rim are people actually playing
Rim sizes have varied a lot through the years, from historic sizes as small as sub 16mm at times to modern commercial offerings reaching into the high 18.5+ range.
Comparing apples to apples is tricky; even for us internally. We don't make all the rims in all the sizes. With the standard 16.5mm junction point getting an R17.0 would be impossible since the radius of curvature is more than the .25mm per side allotted in that configuration. The T can get down to the 17 because of it's sharp bite, but can't hit an 18.25 because there's only 3.2mm to go from the crown down to the junction and the sharp bite demands too steep a slope to get there.
Different brands also have different internal measuring systems. Internationally, we know that european sizes tend to run bigger - a JK or Tilz 18 is our 18.25. Houser tends to run smaller so their 18 might be our 17.75. So to use these numbers across brands is tricky and probably not statistically defensible, but if you do make sure you're controlling for the adjustment.
However, that's enough caveating, lets get down to it.
The next caveat; the 17.25 and 17.0 sizes are probably hurt by not having an R rim in that size. With that said though, the best seller of the T-series is the 18. And even in the E-series which skews the smallest of all at 17.5 as the median size, there aren't too many takers on the 17.25. 18.5 is also hurt by that, so it's possible the true tails of the distribution are wider.
Lastly, we see a slightly weird non-normality with 17.75 less popular than its neighbors. I think this one is psychological, where round numbers good. Maybe if we'd done sizes like "m, l, xl" 17.75 would have out performed its neighbors.
