The invitation to tender (ITT) specified 4 tph Waterloo to Reading all day, but it won't be four of the same speed or calling pattern 2 will be relatively faster than the others.
The calling patterns were left to the bidders to decide, many station calling patterns (per hour) were able to be shared across different routes, and it was also allowable to send some "Readings" via the Hounslow loop. However First have chosen to send all four Reading services via Twickenham, and divert their two extra Windsor services via the Hounslow loop instead.
The end result may be that the Weybridge - Waterloo service is cut back to a shuttle, another option that was given in the ITT - there hasn't been any news about potential downsides to the changes yet. (Funnily enough)
Please don't assume the future timetable is all a great idea by First group, they are basically providing the broad level of service on the Windsor side that any or either bidder was required to do according to the ITT.
The ITT requirements are shown here, however it is in Excel spreadsheet format:
https://www.gov.uk/government/uploa...tachment-a-part1-train-service-provision.xlsx
It is fair to say though that the rolling stock changes are purely down to First.