If the market for footballers resembled a textbook labour market, the players' wages should reflect their "marginal product": the amount they add to the firm's output. If the output of a Premier League side is points, then a player's wage should reflect his addition to the club's points-haul. But if that were the case, then every club would rack up the same points per pound.
Perhaps a better explanation is found in the fact that clubs don't maximize points, but rather payoffs, and there is a big payoff to being the best (e.g., more playoff games, more jerseys sold), and a big negative payoff to being the worst (relegation to Division I).
As a consequence, clubs near the top and bottom are locked into arms races with each other. If one club spends more to finish in the top four, its rivals must spend more also. In particular, it is worth spending a lot to finish first rather than second, as Manchester City did; or 17th rather than 18th, as Queens Park Rangers managed to do. It is worth rather less to finish 11th like Swansea rather than 12th like Norwich City, who finished just below the Swans in the points-per-pound ranking too. That may explain why the clubs near the top and bottom of the Premier league cluster near the bottom of our points-per-pound table. They're not necessarily inefficient or irrational. They're just playing a positional game.