That’s a normal behavior if you are on the stable updatechanel.
In that chanel NC 18 will be earliested be offered with release of 18.0.1.
If you want to update now, you have to switch to the beta chanel. But beware that there have been foundat least one serious bug in 18.0.0 with will be solved with 18.0.1; or you should patch manually.
p.e.: [NC 18.0] Attention! Possible loss of all your shares. Better implement fix now