(no subject)

Having made a complete fool of myself in my previous post I'm writing this with some trepidation. I've finished GoF and have now just started OoTP. And I just wondered.... The ministry of Magic can't tell who (or what) has done a spell because they think Harry did Dobby's levitation spell and presumably that is how the Weasley twins get away with all the Weasley's Wizard Wheezes stuff at home, so how is it that when the order come to pick Harry up in OoTP Tonks does 'pack' and 'locomotor trunk' and Moody does the disillusionment spell on Harry but Harry doesn't get blamed for those spells? Is it simply that the presence of an of age wizard is registered and so the spell assumed to be theirs, and that because of its prejudices the MoM doesn't register the presence of a house elf?