i think different people have different effect on the spells they are using . look at lockhart . he could hardly even disarm Snape, but his Memory Charm is way powerful ( it was his misfortune to have used Ron's wand) but oh well.. guess harry is just really good with Disarming . he used it against Voldie too. In PoA, the strength of hermione's wand+ Harry's anger+ the spell= goodbye snape, good luck.