Missing events when filtred segements


First, a little about our Environment. We have multiple customers that are using the same Application (but different installation) also I track them as one website within PIWIK. This way we can see the overall stats.

In our SPA we track some events with _paq.push(['trackEvent', 'Videos', title); this works as expected.

Now the problem in PIWIK i created a segment for every customer with the following filter:
Referrer URL contains customer1

When I look at the total, I get 27 Total Events / 18 Unique Visitors (All Customers)
Now I go through all Customers and count the events listened I only get 23 Total Events/ 15 Unique Visitors
If I open Segmented Visitor Log and count it manually i get 27/18.

Why do I miss some Events?