IMHO "counter-spelling" is interfering with magic in any way as it is being cast. And "dispelling" is removing magic, and I think I mean both magic already cast as well as magic being cast. Thus "dispelling" is somwehat a subset of "counter-spelling", which can either annull (Perdo most likely), deflect (Rego), block (Creo, to create some blocking medium) or change (Muto) the offensive magic being cast.
So the focus in dispelling would IMHO only apply to the type of counterspell which destrouys or annuls the target spell, not any of the other possible methods.
But I could also be convinced to buy the original poster's definition. Because dispelling a magical effect already there does not sound like counterspelling, so what would I have called that?
And I know of nothing in RAW to support or oppose any of the theories. But note that the text of Winds of Mundane Silence specifically can't be used against Momentary effects, but Unravelling the Fabric of [Form] lacks this limitation. And that the two use different guidelines.