When I was in university and had access to scientific papers, I would often read papers on AGW. I remember reading several times that IPCC models were conservative and the msot bullish models in their assessment reports hadn't predicted the rate at which the north pole is melting.
Does that account for thickness? I could be wrong but I thought that was also a factor in ice extent: some previously permanent ice melted and came back thinner.
Also, 2007 had a much smaller extent than the recent average. Only 2012 was worse.