That's the average for all center prospects. We all know the center position has become extremely watered-down in the NBA. If you look back at the top 10 center prospects over the past decade, I challenge you to find anyone with a standing reach that is lower than Kanter's (9'1.5"). Why pick an undersized, unathletic center with the #4 pick in the entire draft??
In terms of standing reach, Kanter is within an inch (+/-) of Kaman, Okafor, Bogut, Chandler, Aldridge, Brand, Jefferson, Bosh, Nene, Anthony Randolph, Greg Monroe, and Amare. Some of those guys have a much more impressive no step vertical, however, so a lot depends whether you're happy with a positional defender who will contest and block shots with his length and hold his ground in the paint - or with a guy who contests shots by getting off the ground. I don't believe we have any evidence on how high Jonas can jump, but I don't think he's considered very athletic either.
Anyway, the whole problem with this draft is the lack of elite players with the measurables, performance, and skill you'd like to see.
Every selection (beyond Irving) IMO is going to involve trading off one desirable trait for another, or a known for an unknown.
If Kanter can score in the NBA like he's done at other levels and starts putting up 30 & 15's, the Cavs would find a way to compliment him on D.