Yes, that is an interesting observation. You have probably maximized your Form1 before you took its size. Now, what Windows does when it maximizes a frame window is: It increases its size such that the window frame falls outside the screen area. Hence visually the frame disappears. Behind the scenes, however, the window starts at some negative position and is slightly larger than the screen. That explains your observation.
[EDIT] Answer to your additional question:
You can retrieve the size of the so-called workarea by calling SystemParametersInfo with argument SPI_GETWORKAREA. See
SystemParametersInfo[
^].