Sooghuwhoon51
Have you read the information page about saver day passes?
The header under the picture actually already answers your question partially:
book earlier, benefit more
Also, the number of available SDPs for a particular travel date at a particular price point is always limited. So once the (secret) number of initally available passes e.g. for 52.- full fare is sold out, the price steps up to 61.-, again later to 70.-. etc. until 119.- is reached or even those are sold out.
Therefore, according to well-known “supply and demand” principles, SDPs for Boxing Day 26.Dec. may be more expensive than for the Xmas Day 25.Dec., when obviously less people plan to travel.