I believe I have no destroyed a tank with a 100lb or 250lb bomb unless it has been a direct hit. A 500 or 1000 lb bomb is powerful enough to simply blow apart a tank from a close proximity impact. That amount of power can rip right through steel, tearing it apart and utterly destroying it with brute force. The AP rockets (Pb1) from the 190F-8, and the 60lb SAP (semi-AP) from the British planes actually penetrate the armor and great massive damage internally, usually killing the crew and rendering the tank destroyed.
The AP rounds from a tank or anti-tank gun penetrate the armor of the target and either kill the crew or destroy the mechanics of the gun, engine, transmission, turret, etc.