Why obsolete shell tests concealed a gunnery problem

If pre-Greenboy shells penetrated armor plate with inert fillers, did that show their basic construction was sound?

No. They passed trials built around obsolete armor standards, so the tests did not prove they could defeat contemporary protection.

British shells could perform as expected in trials while still being inadequate in combat because the armor plates and standards had not kept pace with warship development. The tests represented threats from decades earlier, so a successful result offered false reassurance. Austro-Hungarian competition between gun makers kept its testing more demanding and exposed the cost of institutional stagnation elsewhere.