I don't know which version the Delta Airbus (Northwest) radars are installed, but remember this: At high altitude, a cell may have ice particles. Reflection of ice particles is weak. An incorrect tilt may lead to only scanning the upper (less reflective) part of a cell. As a consequence, a cell may not be detected or may be underestimated.

Also, Hail is very common under the downwind overhang of a strong thunderstorm - even in clear air. This precipitation which is obviously frozen, is often not detected by even the most modern radars, including ours.