Hitting Zone - why does it change?

So I've had my R50 for almost 4 months, and I noticed this in the beginning, but after it switched, I just didnt question it. But now I'm seeing it again in a few places and I still never understood what was happening. When I got the monitor, out of the box it had a hitting area where there was an angle falling off to the back, but then after playing with the target alignment a bit, it changed to this square. Why does this change? 

Square: https://imgur.com/Q9SRdje

Angle: https://imgur.com/U1V00a7

note: I'm not asking so someone can tell me 'dont worry about it'. im genuinely curious why it changes