The PF's at Mumbai CSMT have different length. Few can accomodate 24 coaches, others just 17-18 coaches etc. Based on empty slot of these long PF's trains have to be adjusted. Another factor being Pitlines, Stabling lines which are shorter & extending them may not be possible due to no space at yard. That may be the reason you find CSMT based trains shorter while other zone, Div based trains longer.