Sum message count is returning different data for analytics api

Google Cloud Apigee
1

I am calling analytics api to generate 2 reports.

  1. sum message count per month for the year 2017.

  2. sum message count per developer app for the year 2017.

When I total the sum message count of above reports I see a difference of 520. I have also consider (not set) in my result.

e.g: sample apis that I used for above purpose

  1. http://mydomain/v1/organizations/myorg/e/prod/stats/?timeRange=01/01/2017 00:00~02/01/2017 00:00&select=sum(message_count)
  2. http://mydomain/v1/organizations/myorg/e/prod/stats/developer_app?timeRange=01/01/2017 00:00~12/19/2017 00:00&select=sum(message_count)

I called 1st api 12 times for each month changing timeRange. last call has timeRange=12/01/2017 00:00~12/19/2017 00:00

Any thoughts on why the sum total is coming out to be different for the above 2 reports ?