List a Project's Issues API Call produces random results

I wrote a python script to retrieve all sentry issues for a project and then print the number of issues where first seen and last seen are more than a week apart (for the last 6 calendar weeks).

Now I’ve noticed the following problem:

When running the script repeatedly at short intervals I get a json response with different items in it. For example: in json response A there are several issues that no longer appear in json response B (executed at a later time).
So I get totally different values randomly, does someone has an idea?

The sentry endpoint: