The 'walk-up' single fare, in either direction, is �64.50.
A walk-up return fare is twice this, at �129, unless you travel off-peak, in which case a Saver will cost you �79.70.
Advance booking can offer substantially lower fares but these discounted fares aren't available on all services. Where discount fares are available, there may be an initial quota sold at �13 single. Once this quota has gone, the next block will be sold at �18.50. After this, the fare rises to �28.00. If all the discounted fares have gone (or if there weren't any discounted tickets because peak time travel has been chosen) the only single tickets available will be at the full price of �68.50.
It seems that you've looked at the fares on an unpopular service outbound but a popular one for return travel.
Train operating companies generally operate a system similar to that operated by the budget airlines. This means that, to get the cheapest fares, you should book early and try to avoid the most popular services. (I checked some flights with Ryanair a few days ago. I was offered �0.79 and �149.99 back. This made perfect sense because of the days and times of the flights).
Chris