Deliberate mistake: After trying to get away from the monster, Costello finally unties the boat as Abbott rows away from the burning dock. The next shot of the boat has Abbott slowly rowing the boat, but you can see it's not moving as there's no wake, and when they jump off the boat after the Invisible man speaks, it doesn't rock, meaning the boat was mounted to something to prevent any movement, so they could make the jumps into the water.
Other mistake: During opening scene, two faces of Big Ben show different times. One face shows 8:00 and the other shows 9:00.
Answer: The monster may have seemed obedient, but its overall behavior is unknown to us. It's possible that it was at times defiant, obstinate, or just didn't follow instructions completely or competently. It's also just a plot device for a silly movie. There has to be some reason, however flimsy, why Dracula wants Wilbur's brain.
raywest ★