Aerospace Butterfly Valves with relative angle between disc and shaft

Hello everyone,

I need some help in understanding a particular architecture of butterfly valve commonly used in aerospace pneumatic control valves (engine bleed valves, anti-ice, PRSOV, etc.).

I see that butterfly valves commonly used in other industries (from concentric type up to triple offset type) always feature the shaft parallel to the valve disc.

In most of aerospace high performance butterfly valves (pressure greater than 300 psig, temperature up to 1200°F) the shaft has a relative angle ("gamma" in the image below) with respect to the valve disc. The valve disc plane is of course perpendicular to the valve axis when the disc is closed (in the left image the disc plane is perpendicular to the seat axis but the concept of the relative angle between shaft and valve disc plane is the same).
Why is this angle "gamma" introduced?


I believe having this angle "gamma" is a way for implementing an offset but I'm not fully sure and I'd like to hear for someone more expert than me.

Thank you in advance.

Matt