It's an encoder, not a potentiometer. This means it's "endless" - they made it just transmit "the knob moves right" and "the knob moves left". By having it transmit from 0 to 127 (regular MIDI CC) you'd be limited to 128 steps. This becomes an issue when you have something like an EQ/filter (because with the slightest touch of your knob you'd jump like 100Hz and everything inbetween would not be accessible), and even more important when you want to map it to song browsing in DJ software (because you could only choose 128 songs then). You'd need to script this (I suppose Bome Midi Translator should be able to handle that) and translate from 49 = moves left, 50 = stands still, 51 = moves right, to whatever minimum/maximum value you prefer/require (could be even 0-127 and sent out to your DAW as MIDI CC)

Edit: This is btw how all encoders work, the only thing is they usually have the mapping done in firmware and send out values 0-127. So what the guys at Behringer did is both nice when you need it and inconvenient at times.