Allan,
It is not unusual to see productive links at 40-miles on 900MHz due to the RF propogation. I suspect your results are reflection of configuration settings on LINKPlanner; and or a fully obstructed path being planned.
You can find a good webinar on LINKPlanner on the Forum at http://community.cambiumnetworks.com/t5/LINKPlanner/Planning-Networks-with-LINKPlanner-webinar-replay/m-p/48155#U48155
If that doesn't do the trick we can can connect you to a Regional Technical Manager to assist with familiarizing you with LINKPlanner.
Scott