The timetable change occurred before platforms 5 & 6 reopend and thus when trains had fewer platforms to choose from.
The timetable change in December was based on the work being finished. Of course it wasn’t due to be done until the Christmas period, so there was a timetable variation put in place for the first couple of weeks from the start of the timetable until the work was due to finish, which replanned the platforming. (Albeit this all had to be ripped up for the strike timetables). However the work hit a snag a couple of months back which delayed completion, so the variations were carried forward for the first 2 weeks of January.
Now that the platforms are open, the variations have been discontinued, and the base timetable applies.

