Interesting question; I wondered that aloud when I first saw the 'heads up' years ago (regular Limelight gets quite large).
I have only seen it for sale here for the last 2-3 years but it does not appear to be in places/homes that I frequent because I have yet to see one in a home, park, city garden/etc. Thus, I am not sure how those specimens have done.
My take on those estimates is that they are a guesstimate when the plant is 10 years old (or maturity, as they call it in some plant labels). When there is a range given, I assume that growth will be limited in places where the growing season is short so, in the northernmost states, we might see growth of 3' and in the southernmost states we might see growth of 5'. LL has not been for sale that long yet so it should be interesting to see if it is indeed a dwarf or just a slow growing, regular sized, plant eventually. I remember a magnolia variety once having that outcome. Was it Little Gem maybe? I can't remember?!?