The 17 Pounders "High Velocity" guns are actually just fancy tracers, basically the same entity you see from rifle fire. Looks cool as heck but unfortunately also means they can not hit anything except ground. If it rolls a miss, it's a miss nomatter where you see the tracer go, unlike with actual shells.
maybe that is a good thing. I think you can still kill smoked tanks by hit ground? I have seen some funny Esports videos with paks killing random stuff in the base(Panzerwerfer, Ambulance....), because they have shells