Yeah, it probably is, what with the Thundergun and sh*t. Still, excluding NTZ and the smaller, non-maps in Green Run, the point still stands.
Most maps on BO1 are much more difficult than than those on BO2, and this is reflected by the scores some people can register when playing co-op.
Moon and Ascension are clearly very easy maps, but I still find them more challenging than say Tranzit, Die Rise and MotD.
This is in no way me being disrespectful to the individuals I refered to who have lower scores on BO1 though. I too am one of these players who have vastly superior scores on BO2, although my BO1 scores are not too bad.
It is more that the maps and training areas are smaller and tighter than the BO2 maps, so this tends to show up in players scores.