Maybe it because we have negotiated contracts and contract changed for more players than any other club. Players coming in, going out and signing new contracts/extensions all involve agent cuts... plus loans both ways too!. Seeing as we have done the above so many times, it perfectly natural that we would have spend more on agents than other clubs.
As for the return, I cant say for sure but I would assume that by moving some players off the book either on loan or permanently has helped us stay fluid for a while. And of course, when they go out, you need to cheaper options in. So if spending a few hundred thousand on agent fees has meant that we have saved a few million on wages, the return (on a purely financial basis) as been OK.