button_press_event & gdk_pointer_is_grabbed()

I noticed that doing a gdk_pointer_is_grabbed() in a 
button_press_event callback on linux produces FALSE while on win32 
TRUE.  Looking at the win32 source code I see that a gdk_pointer_grab 
is done in gdkevents-win32.c, under the comment /* Emulate X11's 
automatic active grab */.  What is automatic active grab, and why 
is the pointer not automatically grabbed in the X11 code?

I wonder if this is related to post #3328 "Button-release event"