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Zipline Output

Hi,
I am using zipline for running a strategy. In that strategy, by default benchmark is "SPY". For running the zipline algorithm, I am using following
command:
%zipline --bundle quantopian-quandl --start 2000-1-1 --end 2012-1-1 -o backtest.pickle

can you explain how benchmark period returns are calculated in the output given by above command and if that returns are cumulative returns , then by using the formula as cumulative_return(i) = product (1+ return(i)) -1 , not matching with benchmark period returns given in the output of zipline