I suspect part, if not all of the issue is most of the time delays caused by a 'train' operated by an infrastructure company, such as Balfour Beatty, are going to be coded the same way, "MV", there simply isn't the variety there as for other operators, if it was a TOC or FOC individual causes are more likely to have different coding. Late start, MV; time lost in running, MV; driver late, MV; seized axle on an RRV, MV etc. As "M" delays are broadly mechanical it then becomes like an extreme case of RTT's grouping of some reasons for trains being cancelled: failed engineers train.