Okay, BUT: The night he went into Ron's room with the knife, that was the night he used the stolen passwords to get into Gryffindor Tower, wasn't it? There were two incidents--the one where the portrait got slashed (and Sirius didn't get into the Tower) and then the one you quoted, where he DID get in, but using the passwords. Right? Not right?
So it's conceivable that he went in once as Snuffles with the goal of just chomping on Scabbers, but when he couldn't get past the portrait, he had to try a 2nd time as a man, with the stolen password (and the knife).