Why wouldn't the angle markings on an attitude indicator match the horizon line?

Question

This is a video of some guys messing around in a 737 simulator. Starting at about 1:13, when they have the plane in a pretty deep stall, the attitude indicator degree markings no longer line up with the blue-and-brown horizon line. For instance, at 1:15, the markings indicate that the airplane is about 55 degrees below the horizon, but the horizon line itself is only about 20 degrees above the indicator.

So, which one is right? Is the plane 55 degrees or 20 degrees nose-down? And (most importantly) why would the attitude indicator give two conflicting indications like that?

Screenshot from the video at approximately 1:15:

Answer 1

The degree markers are correct, and it's done to improve the readability of the instrument.

If the horizon line was "accurate" at that moment, it would be off the screen and you would be seeing all brown, because the indicator only spans about 45° from edge to edge. Instead of allowing that to happen, the horizon line simply stops moving off the screen when you get much beyond 20° of pitch up or down, so that you always have some blue and some brown to figure out "which way is up". You can see it happening pretty clearly at about 0:34 of the video (when things aren't moving around so chaotically yet). Before that point, the heavy white 0° line coincides with the division between blue and brown, but as the plane continues to pitch up, the 0° line keeps sliding down off the screen, while the blue/brown line stays parked near the bottom of the screen.