Automated 'PrtScn' keystroke acts like 'Alt+PrtScrn'

We need a screenshot of our app for a unit test. CaptureScreen() and CopyFromScreen() somehow ignore the app and return pictures of an empty desktop. So we wrote this to fake a PrtScn keystroke:

public static Bitmap GetAltScreenshot()
{
    Clipboard.Clear();
    SendKeys.SendWait("{PRTSC}");
    while (!Clipboard.ContainsImage())
    {
        Thread.Sleep(500);
    }
    return new Bitmap(Clipboard.GetImage());
}

Alt isn't part of the keystroke, so this should return a bitmap of the entire screen. Yet somehow this snippet returns just the focused window. Which is fine, that solves our problem - but we don't understand how.

Why does this return a shot of just the focused window, instead of the entire monitor?