If they can't get even this easy stuff right, what confidence should I have they get the complex systems right behind true RNG or that what they advertise is indeed true at the math level?
Circles represent intersection of bullet spread cones and the screen surface.
They use an unrealistic model where angles at which bullets are shot are spread evenly within the cone, hence diameter for 100% is exactly 2x bigger then for 50%.
Truly realistic model would have a nonlinear distribution taken from empiric data ( Gaussian ? ), where most shots are concentarted closer to the center but there are some outliers around ( random muscle shakes, distractions, bullets with out of tollerance parameters, etc. ).
So realistic model would have even smaller 50% circle not bigger. Then each subsequent shot in the burst mode should have the center of the probability cone randomly kicked around, but the devs don't aim at being a military sim anyway.
What you posted on the other hand is just bullshit with no basis in physics, or math, because diameters are proportional to bullet spread cone angles, not surface areas.
Your picture just represents a failure at understanding and abstracting a high school level physics problem into mathemathics.
This model would shine the most in a realistic 3d environment, with free movement and unit stances.
In super abstract environment alligned to a cube grid, with chest high cover boxes, it's a bit of an overkill, but it's an easy and accurate way of implementing aiming at and hitting individual body parts I guess and they wanted this feature to stand out from other xcom clones.
Random note: The way they first play shooting animations and bullet hits and separately getting hit and damage numbers later is so awkward. They should calculate everything under the hood first and then display it simultaneously.
It does not have to work like that. That depends on weapon. Maybe the weapon has a real problem shooting straight, in that case 50% will not be perfect 50% but more wide. Or it is very precise weapon and then 50% will be smaller circle.
Rather single shots should be more concentrated, while burst and auto modes should be less concentrated to abstract gun being kicked around. Gun accuracy should just scale the circles.
My guess is they will just have an even distribution for all weapons and fire modes, while weapon and eventually unit stats will just scale the circle.
It's a casual game, with no ambition of being a military sim and the only reason they have this at all, is because they wanted individual body parts hitting to stand out from other nuXCOM clones.
No matter the motivation and whether it really adds much in this particullar game, it's still more accurate then any other XCOM clone.