The reason is a broken camera hook. It seems that the number of arguments are different between wg and lesta client. I'll try to fix it
Based on the fact that the penetration power of an ap/apcr projectile is not only losing over distance. It's also affected by a random value. It's +/- 25% of the base penetration power.
The mod can't know this value, but it can calculated the remaining piercing power over distance and can subtract 25% of this value. When this value is still greater as the enemy tank armor, it will for sure penetrate the armor. At least if you hit exactly the point where you're aiming at (dispersion factor).
Penetration example:
The default alpha pen for the Object 268 V4 is 293mm. This value will decrease over distance
-25% = 219mm
+25%= 367mm
The RNG roles a value for your shot penetration between 219mm (-25% lowest) and 367mm (+25% highest) as its also shown in the tooltip
Return to your main questions: RNG stands for Random Number Generator, that calculates the value.
I can add russian language if someone will help to translate it.