A little difficult problem to communicate without actual data, but let me try.

I am using Live.getLastVisitsDetails endpoint to get the visit details. I am using offset_limit for paging the details shown. It works as intended. I am adding fliter for users with events. I am using segment with eventCategory. Now if I query the API with offset_limit as 10 and eventCategory==General, it returns me only 6 records. If i only change the offset_limit to 20 it returns me only 11 results.

So, somehow the API is limiting the number of visits and then applying the segment filter, which in turn makes the number of records return incorrect.

Edit: I see that piwik 2.13.0 RC is released just yesterday. I am still using piwik stable version. Not sure if this is fixed in the new release.
Update: Updated piwik to 2.13.0 RC, still same behaviour.