That's because Coinwarz gives you the picture based on the current market conditions but in reality while mining your coins have to be confirmed and that generally takes a long time.  By the time your coins are confirmed the market will probably have corrected itself.  You have to look at other factors like market depth and number of confirmations required before making a decision.

Also, why would someone lease you a rig for less than it would bring him if he was using it for himself?  You're leasing, the overhead pays for the hardware, electricity and obviously profits.  You either buy yourself a miner or you pay someone else a premium to be using his, that's the name of the game.