That all just makes it more astonishing. Arsenal achieved their unbeaten season in a year when the chasing pack was much closer to the eventual winner than this year.
Not sure I agree with that - if you are being pushed you have to keep winning (or rather not losing) if you are miles ahead I'm sure it is pretty difficult to keep that momentum going when you've got plenty of elbow room to lose or draw a couple without being drawn back into a scrap. Just look at last season's astonishing run in with Man City on 98 its just pipping Liverpool on 97. But the key thing is that both just kept winning - Man City won their last 14 games, Liverpool their last 9.
More or Less did a statistical analysis of Liverpool's performances this year based on performances by individual players. They found that the current situation is pretty improbable and "on average" Manchester City should be much closer than they are, but things have just gone Liverpool's way. I suspect that things went Arsenal's way to an even higher degree in the Invincibles year.
I think there is always an element of luck in these matters - luck, in your favour, is what turns a good season into a great one, and conversely turns an OK one into a terrible one.
And although Arsenal's undefeated season is incredible in a symbolic manner, being undefeated is no guarantee of success - indeed, theoretically a team could go unbeaten all season draw all 38 games and be relegated on 38 points.
So for the sake of arguments lets assume Liverpool win their last 10 games to finish on 109 points from a possible 114 with a record of 36 wins, 1 draw, 1 loss. While they'd not have the 'unbeaten' symbolism of Arsenal could you really argue that Arsenal's record of 90 points from 26 wins, 12 draws, 0 losses is actually better. Personally I don't think so.