please help in spi code

 I am working on Comprehensive Monitoring and assessment of Drought in India. For this work I calculated the SPI time-series for Ganga basin with GPCP precipitation data from 1980-1987. I downloaded  matlab SPI code from Matlab Center and am using it. I used this code  with 3 month scale and I compared it with SPI-GPCP 3-month data from IRI / LDEO climate data library for the same basin, but unfortunately there was a huge difference in comparison for some months. I can't understand why your code value is so different from this data.please help

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