differential signals and laser projection range

[shihweichieh2023]

Hey, I have built my own system with the same setting as here since 2013 and it works perfectly. I use MOTU mk3, mk4 as my DAC interface. However I recently found LaserBoy correction board and I found it ground all the - inputs of the ILDA port. Can someone explain why? isn't the voltage range created by the differential signals were there for the laser projection range? or is it only for the signal losing prevention?

https://laserboy.org/forum/index.php?topic=561.0