Someone (not me) has spotted that the 498 times on one part of the TfL site
for school days and Saturdays are quite different from those on another part (the working timetables)
It also means that the school day times differ throughout the day from the school holiday times.
This seems to come from a change made to just school days and Saturdays in February 2021, which my contact tells me they assumed was just a temporary Covid-related change.
Anyway, whichever is right, they should match up for this low frequency route.