Is it possible that this is a result of a binary overflow? Some binary variables of size 0-255 are used to describe values from -127 to 128 instead, thus if a value goes over 128, it comes into the negative range instead (had this with a stealth guy in Jagged Alliance 2: When his skill became too high, he was noisy as hell instead of being super stealthy)...

My idea comes from my extensive use of advanced materials on those boltguns, so the result may have overshot the upper limit and become negative instead...