Buxton doesn't even have a franchise commitment for 2tph. Knutsford and Northwich do, for December 2017. Surely you can understand why people see this as bizarre.
After the May 2018 timetable fiasco, I do not find it bizarre, or even surprising, that TfN, Network Rail and Northern are now offering, for May 2019, only timetable enhancements that can be reliably delivered with the existing infrastructure. It appears that some of the enhancements specified in the Northern and TPE TSRs were based on preliminary Northern Hub timetable modelling that has turned out to be flawed and over-optimistic.
The franchise agreement specified 2tph from Manchester to New Mills Newtown, but only 1tph to Buxton. But there is no crossover to enable a reversal at New Mills Newtown. Presumably Northern has found that it can extend the second train to Buxton at minimal additional cost and thereby gain additional revenue.
The franchise agreement specified 4tph from Manchester to Hazel Grove. Presumably Network Rail has now determined that it cannot offer a path for the fourth service without unaccceptable performance risk. So TfN and Northern have agreed that it the third path should be used for the second New Mills Newtown/Buxton service rather than a second Hazel Grove terminator.
The franchise agreement specified 4tph from Manchester to Levenshulme and Heaton Chapel, of which 16 per day must come from Bolton. Providing a cross-city service between Blackpool North and Hazel Grove, as in the pre-May 2018 timetable, will enable these commitments to be fulfilled.
The franchise agreement specified 2tph from Manchester to Macclesfield. But they would have to be squeezed in between the 2tph VTWC and 2tph XC expresses on the two track main line, and across the flat junction at Cheadle Hulme. Presumably Network Rail has now determined that it cannot offer a path for the second train without unacceptable performance risk.
The franchise agreement specified 2tph from Manchester to Altrincham, Hale, Knutsford, Northwich and Greenbank, but only 1tph to Chester and the other intermediate stations. Presumably Network Rail has now determined that it cannot offer a second path between Stockport and Altrincham, over the two single line sections, without unacceptable performance risk. So TfN and Northern have agreed that the second train should instead run from Altrincham to Chester, with the frequent Metrolink service providing connections from Manchester to Altrincham.