A timetable change twice a year might not seem very often, but, equally, loadings on a particular journey are not inclined to vary from minute to minute anyway. If a particular journey gets to the point where it needs attention, whether that by by way of adjustment to the timetable or capacity enhancement, then the likelihood is that the situation has been developing for a year or two.
Clearly you aren't familiar with school holidays, the mad September return to work and the concept of university holiday break ups - all have a huge impact on journeys fro regulars. Loading on a peak hour train in October is very different to that of the same timed train in peak hour July.