I think it's because the Premier League has become much more competitive over the past couple of years.
Spurs have emerged as not shite, and Man City have spent hundreds of million. Before the top 4 was pretty much guaranteed every season, to be United, Chelski, Arsenal and us.
Teams have an overall harder season now, and play against better teams, which takes it toll.
We have been better defensively than Spurs. We have conceded 2 less, and we conceded 4 against Spurs alone in one game. :p