I've noticed this too.. Playing both against and as Ostheer. They're too accurate it seems. Have they been buffed recently? There've been so many patch notes recently I haven't read all of them.
All AoE weapons were reworked to always pen and never miss in exchange for smaller AoEs.
Example: Basically, before, you could have a mortar shell fall near an entity and he would take no damage as the blast would 'miss' him or (in case of shocks/vehicle) possibly not 'pen' him. They got rid of that non-sense and tweaked the AoEs to be smaller for all AoE weapons.
Hence now the mortar and all other AoE weapons do their dmg reliably. Its just more noticeable on the Ostheer mortar because of its (current) insanely high RoF which will soon be nerfed to a (possibly) more sensible value