Well it shouldn’t be a straight accumulation (aka addition) because a positive value can compensate a negative one perfectly, drawing no arc at all, while the modulation actually moves following the combined value
The range display covers the min max value while the modulation value combines it.
If you have two modulators, one set to -25% intensity, the other to 25% intensity (both in unipolar mode), and the value is at 50% position, then the modulation range will cover 25% to 75%. If both modulators crank out 1.0 as mod value, then the modulation value will sit right in the middle where the value is supposed to be.
That being said there might be a few edge cases and combination of modes & intensity where the math doesn't math, but in general that should already be the case.