http://www.spotifyartists.com/spotify-explained/#royalties-i...
"They absolutely don't pay every artist the same amount. Besides the fact that per-stream rates are almost meaningless because they vary month-to-month as Spotify's own revenues fluctuate, I would be willing to believe major labels get a higher payout as a result of their equity positions. And with that, it isn't too preposterous that Atlantic (via parent WMG) would have brought Led Zeppelin exclusively to Spotify on special terms such as a minimum per-stream rate, some free extended term advertising to artificially boost data figures, or something entirely more enticing. And even if Spotify is, in fact, paying out an equal rate for Led Zeppelin streams, it may be that Atlantic received a nice advance--or even a debt-free bonus--as an incentive."
So maybe Spotify is dealing with the labels/publishers (Atlantic has Zepp) to cut this special deal and perhaps allow them a bigger rev share?