Dive planning with subsurface

[Willem Ferguson]

I am trying to get to grips with some of the more technical dive 
planning features of subsurface.

Doing a dive using the NOAA no-compression Air table, I should be able 
to dive to 34m for 20 minutes and return to the surface without a deco 
stop. I set the GF at 40/80 and now plan this dive using the dive 
planner, enabling the ceiling calculations. What happens now is that the 
dive becomes a deco dive with three deco stops, starting at 9m, because 
the ceiling does not allow me to surface safely. See screen dump:
https://www.dropbox.com/s/7by4ecqjwgr6f13/PlanDiag1.png

The data for the dive are very simple:
GF40/80
34m  +3 min
34m  +20 min

What am I missing?
Kind regards, Willem