Switching Focus between opened Windows

Hi there:)
 
Can somebody confirm this:
 
1. Make a Midi Track
2. Open a Synth
3. Open PRV extra Window (Not in MD)
 
Now the Focus is in PRV, the Menu letters are black.
 
Press Ctrl + TAB
 
Now the Focus is in the Synth Window
 
Press Ctrl + TAB
 
Now the Focus is in the Track Window
 
--->> With pressing Ctrl + TAB again you now can't give the Focus back to PRV. It stays at Track window.
 
Why?? 
 
Thanks a lot:)
Bassman.