ERA5 Monthly drought indices

Hello Everyone
I have been working on ERA5 Monthly drought indices and the dataset i get from it contains some unrealistic values. while the realistic range of the dataste lies between -3 and +3 (though infinity on both sides, but obtaining values across these range is severely unrealistic) but the values i got formony months is -8.22 for different categories of spi. Like May 1984, the spi1 values for this month is -8.22, similarly i have many months from 1978 till 2015 that contains similiar unrealistic values. Is there a possibilty that i have committed a mistake or is there something really wrong in the dataset. Kindly guide me for this case. Thank you