Window is smaller than requested in CreateWindowExW()

I wanted to create a 300×100 window with CreateWindowExW:

HWND hwnd = CreateWindowExW(0, WndClassName, L"300 x 100 window", WS_OVERLAPPED | WS_CAPTION | WS_SYSMENU, CW_USEDEFAULT, CW_USEDEFAULT, 300, 100, NULL, NULL, hInstance, NULL);

But, I got a 286×93 window. Why?

Furthermore, when I create it in position (0, 0), it is actually created in (7, 0). Why?