I always found this surprising. Now, maybe I have an explanation. Please look at this thread :
In message #3, I give some information about the window hierarchy implemented by DPL. I think that those of you who are developers might be surprised, especially when comparing with Lightroom. I have the feeling that the DPL main window is actually a big dialog box.
I discovered these details by chance when trying to answer a question from user @Joaquin.