The ticket price being different is an obvious reason why a percentage tax would change!

I'm sure I've seen before that some schemes calculate these taxes by charging what it would cost if you were buying the ticket with cash that day (rightly or wrongly). There is indeed a big difference in cash ticket prices on those 2 days, but it can't be 5% at the prices quoted.

EDIT: It's £26 on the cheaper day and £44.70 on the more expensive day, which doesn't fully explain the £36 difference. Maybe there was an error somewhere?