It's more the comparison of playing at a higher level than to be expected. Right now Steen is on pace for a 90 point season, which will not happen. Any season prior he's put up the most of 50 points, which is more about the level he is at as a player. Clarkson won't produce offensively again like he had for the contract he was paid, and Steen will never reach a 70+ point season beyond this one if he does hit that mark.
Comparing the two last year, Clarkson and Steen were separated by only 3 points, which in a full season would have been 6 points. I know they are very different style of players, but the contracts that were given to both were based around their highest career level of play that likely won't be matched again.
I could be wrong, but I think Steen is a career 50-60 point per season max player as Clarkson is a career 30-40 point per season player. Steen is essentially making one million less per season now as Datsyuk and he is nowhere near the same level of player. Again, just my opinion, but I think it's an overpayment.