In my 4 play-through's of RtB as Allies, my mortars are what I'd expect. However, the German side's mortars are sometimes wacky. The last play-through they fired-for-effect on top of their own troops. The spotter was most likely very much suppressed and that might be why. The 1st time I played this scenario, I lolly-gagged around and got hammered by the German mortars.
So, I tend to think it has to do with LOS and suppression...i guess.
FWiW