I interpret it like this:
Spells have have different "intensities" of their effects. Just as in CoS Snape used expelliarmus to disarm and knock back Lockhart, probably because he doesn't like him. He only disarmed Sirius because he didn't want to harm him, but take credit.
Harry used the same spell but it was intensified by his anger, just as his patronus becomes stronger with a stronger memory. The fact is it still disarmed the opposer, but with different intensities it can have stronger or weaker effects.