My beds that get some shade, from the hot afternoon sun, always grow exceptionally tall stalks.......not as many as the ones in the sun....but they're taller !
As to the "registered height", I would suppose that 30" was the average of the stalks , in the hybridizers garden (as well as anyplace he happened to send plants , for "test"...if he did), and, it would be just that, an average. My point is, reading the "registered height" on the wiki, or , for that matter, the "listed height" on a sellers page, or catalog....is just a rough idea of the iris' true height......different soils, different climate, different culture changes a LOT about how an iris grows.....and height seems to be the thing most affected. That, coupled with the "possibility" that , when "describing" the iris' height, coloring, season, or other "attributes", the hybridizer (read that as "proud parent" !) is very liable to "gild the lily" (or, in this case, the iris
!) just a bit....and , really, so what !! If an SDB happens to grow on the "tall" side, maybe it can be registered as an IB, and if the TB quite often ends up "short"....let's call it a BB. The only problem with doing that is when someone wants a row of "mid season, 30" tall" iris, and right behind that, a row of "mid season" 38" iris, and when they all get into bloom....there are tall ones in front, AND in back, and the season of bloom runs from "early," to "very late"....ten they are disappointed ! That's the reason , in the registrations, the AIS wants that information, as a guideline for those folks who want to 'plan their garden'.....in reality...it's all still pretty much a guessing game, as all that really counts....is how it grows for you !
It would probably grow in class for me. I can rarely get TBs above 30" in my growing conditions even when I have the rare good year for them. Something about my spot in the world because I can give them to someone 10 miles away and they will manage to reach the correct height 
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. Could be the grower. Generally I do better with getting arilbreds to match the official registration specs, but not so much other bearded iris. Mostly the medians just haven't survived well and those that have don't bloom very often. IBs have performed the best. The medians tend to do better after a colder and wetter than normal winter. Often our winters don't provide those conditions. TBs do pretty well a lot of the time, but just tend to be shorter and with fewer buds than they have for others. Spuria iris reach good height most of the time. I used to spend a lot of time trying to figure out why the TBs didn't do those things very often, but I gave up finally and just take what I can get. 
