Hmmm! Conumdrum!
Are you using an offer code of some sorts? I wonder if it's not been correctly linked to Daytrip fares. Going to the homepage,
www.myferrylink.com, and setting both dates in the quote to the 4th April and clicking the red quote button gave me the £25 price.
Tell you what, if you've found a bug, you can have a daytrip on me!