Hi, I'm just wondering why there is a size difference between a widget in a window

e.g. in design viewer it's 600 x 450

and when I write this->width(); and this->height(); I get 596 x 456.

Doesn't take a rocket scientist to figure out that there is a 4 pixel difference there (is that always going to be the case???), however I'm failing to understand why it's there in the first place? Just accept it, account for it and move on?