The returned number of "open events" for emails do not match the ones on the user interface

We request all the events from

https://api.hubapi.com/email/public/v1/events

for a specific date range.
After requesting all the available pages we find the unique events with the type “open” and count how many are there.
We expect the number to match the number of “open events” on the user interface, but it differs quite a bit.
Is there a better way to get this number ?
Is it a bug if the two values are different ?