Man I don't know...assuming someone is reasonably lean and not shrek status height, I can't imagine a lot of folks are in that 18-20 range.
Then again I'm not even tall enough to get the ice cream out of my freezer and have rabbit like proportions, so I don't need to live in a world where >17 is necessary.
But for real, even if someone is beyond 17, the 17" cuff is still going to be suitable for most people. I know at least a couple people over >17 who get congruent readings with the 3 series and the $3000 wall mounted unit at their doctors office.
Anecdotally, one guy I know with arms around 19" has no issues with the 3 series.