My understanding is that mountune does custom calibrations based on your mods and different datalogs captured with your AP that you send them via email and they create a calibration based on your specific vehicle etc. I believe this is randy's specialty (he used to do the same thing at FSWerks i think). Cobb's OTS maps are much more general and therefore not optimzed for your specific setup, just something they can sell to work somewhat well with everyone's car (hence they are free).
However today only, a custom calibration from mountune is free if you buy an AP from them. Already placed my order haha