Inconsistent Segment Figures

I have problems understanding the Segment Figures
When I compare Segment figures to the Total, I get inconsistent numbers.

example: I created a segment showing all countries request from CH: Country is CH
and I created another segment showing all request from countries NOT CH: Country Is Not CH

The Sum of these 2 segments is not equal to the Total of all requests
The difference is quiet huge, up to 40%

Any ideas on what is causing this differences?

Grateful for your responses