Total page views by month is different from monthly combined numbers

Hi,

When I check total number of views monthly for each month of last 6 months and I add them manually it is different from total number when I run the report combined for 6 months.

When I run report for 6 months Jan to June together it gives me 2.5M views
When I report for each month separately for each month of last 6 months and add them myself I get around 3.2M views

Do you know why there would be such a big difference in numbers ?

Thanks for your help