Yes, I agree. Rail service in the UK isn’t the same at weeekends. I did cover the subject of the quirky East Coast pricing a few months back.
For some trains it can be cheaper to book first Advance rather than second Advance.
Today, for example, (when I checked on Sunday) a one-way first class Advance from London to Edinburgh with the 0800 train cost £99.50 whereas the standard class Advance was priced at £139. That’s a difference of almost £40 which is silly.
It’s all to do with price zones selling out and the rail booking system (all the TOCs use a single system) being slow to readjust.