Were you performing an autoland or just landing with the auto throttles on but flying manually? If you were flying manually, the auto throttles won’t go to idle because they aren’t designed to. The only time you should leave them connected is if you are performing an autoland. When manually flying the 757 in any phase of flight except takeoff and climb, the auto throttles should be disconnected and thrust should be manipulated manually. So long story short, the sim is correct. 8-)