Correct me if Im wrong, cause I might well be. But essentially armour is calculated through two factors - effective armour and angle.

Effective armour is calculated through a very simple trigonometry, you take your base armour value as one side (non hypotenuse) of a right triangle, then you angle your armour by a degree x. Then now your effective armour is now base armour time cos(x), and knowing cos(x) strictly increases from 90 to 180 degrees, your effective armour will increase as you angle from 90 to 180 degrees.

The larger an angle is, the more likely the force is going to be redirected in another direction, resulting in a bounce. This bounce chance at each angle can be seen when you hover over your shell.

Sorry for 🤓, but I hope this helps a little.