Advertised size compared to true spec size?

[JoJoSilverado]

When I was shopping for tires, one thing that caught my attention but never really inquired about, (not that it is going to affect me as I've already made my choice), but why are 33x12.5 sized tires not a true 33x12.5? Same with 35/37x12.5

 

Like, the width is usually only .02 off, if that. But why is the height of the tire on those measurements almost always ~0.25 shorter than the stated size? ie: 32.76 (33x12.50), 34.76 (35x12.50), 36.77 (37x12.50)

 

Like, if you want a true 33x12.5, you'd probably be better off going with the 305/55 size (this is with a 20" wheel of course). But some people like the simplicity of their sidewall tire size being the easy inch-based numbers, instead of metric.

 

The metric version of a 33x12.50r20 tire (305/55/20) would technically come out to 33x12.00 if using true metric-to-inch conversions, but manufacturer specs seem to always bring it out to something along the lines of 33.03x12.44

 

Does anybody know why they don't manufacture these tires true to size? It can't be because it's "not possible" for some reason, as most of these companies that offer a metric version (305/55/20) seem to spec out to almost exactly (and closer to) the true 33x12.5 size for example....?