Why do the faders in Sigma not display exactly the same value as my DAW software faders?
Sigma interprets values according to the Mackie Control protocol, but the values received are entirely determined by the DAW and DAWs are not all exactly the same in terms of how they implement the protocol.
In any case, please bear in mind that Sigma is an analogue device, and therefore going over 0 dB in Sigma does not have the same meaning as in your DAW. Within a DAW, going over 0 dB means unpleasant clipping (although it's not noticeable if it only lasts a very short time like a few samples), whereas in Sigma going over 0 dB will first result in pleasing colouration of the sound.
So, the way to proceed with Sigma is to refer to your ears first, and not focus on the dB values numbers in your DAW. And do not hesitate to push the Sigma faders over 0 dB if it sounds good to your ears.