At approximately 17:30, yes, you're right, and that does make sense. The Liverpool Street to Ely (17:21 from Cambridge) no longer calls at Waterbeach, and the 17:29 to Norwich does. But this leads to the obvious follow-on questions:
- why lengthen the services in the evening that used to call at Waterbeach, but not in the morning?
- why lengthen services if doing so means dropping station calls, so there will actually be fewer passengers on the train! (OK, maybe it needs more capacity between London and Cambridge, rather than between Cambridge and Ely)
But an hour later, no, this can't be the explanation, because the Liverpool Street to King's Lynn (18:20 from Cambridge) does still call at Waterbeach, followed by the 18:30 Norwich train with it's additional call.
The old 18:22 Cambridge to Ely shuttle has been removed completely from the new timetable, and that used to call at Waterbeach, so you could say that the 18:30 Norwich train's additional Waterbeach call replaces that, but nothing has replaced the full journey of the 18:22 to Ely, so there is actually one fewer train to Ely in the evening peak as a result.
It all seems a bit random!