Tickets go on sale November. 5 months since last festival. December and the xmas nonsense still takes up funds for many people- if you are pulling out multiples of 400 in November then that's a massive hit. If it's multiples of 75 people can mitigate it a bit then go full in between jan -April to save the rest.
Some people live pay day to pay day and although on paper it should be one pot - reality is for many life gets in the way and glastonbury Tickets are not priority at that moment in time. A few extra months of wriggle room cab make a massive difference.
Also wasn't it something to do with vat/tax years to split payments?