I suspect this is what happens. Scipt checks for UA in the order (just an example):
IE6
Mozilla
FF
Opera
Safari
Script checks for IE and it becomes positive since MSIE_6.0 is there. It ignores the rest of the string, and goes on to the next UA... And it never reaches the opera test.
If you place Opera check at the begining of all checks, it will always come positive first and will be registered as Opera.
Just a speculation since I don't know how your script actually works.
