'I'm sorry',,Shaw..placing David's head in a bag..

As Shaw is preparing to close the bag,,with David's head..she says..'I'm Sorry' At first I thought it was a little humor with Shaw feeling sorry to close the bag over David's head.. But then ..since she says the words right after speaking.. "You're a robot"..it occurs to me that she was sorry that David could not understand the human feelings/soul/ reasoning for seeking out the Engineer's home.