I agree with cakes,
Michael probably felt he had a good understanding of the situation from what he heard over the phone call. Plus I'm certain
Kimmet had him doing other shit in those 11 days. I doubt he was just sitting on his ass playing Gameboy. He was likely on more missions in nearby areas, scavenging, and the like. In my opinion, he probably bumped the written report down on his list of priorities. He and
Carl had a little tiff over it because Carl did mention that
Robbins was shooting other soldiers that were turning, but the whole phone call was stressed so some details might have been lost. It was a just a miscommunication, but I sensed that Michael was in the wrong and got a little upset and defensive.
Haven't you ever known you are wrong yet you still get upset that someone calls you out on it?
Tanya saw through the bullshit and knew it was pointless to argue. She seems to understand Michael pretty well now. I say kudos to Tanya for getting the conversation back on track.