Because no one looking for market index returns would hold Zynga in anything other than an index fund. If you hold zynga you're looking for a homerun, or a trader.
Thus I used numbers that would appeal to the type that might hold Zynga stock. The type that had a risk profile involving losing 90% of the stock's value in 6 months.
Big error. You can predict a stock doubling in value a year in advance?
Your numbers are less shocking when you plug in more reasonable expectations, say 6% market index instead of 200% home run.