I'm presuming you are trying to buy a Super Off-Peak Single ticket?
Technically you are already being offered a hefty discount on this ticket of circa 50%; the Super Off-Peak Single ticket is actually £141.90 (£93.65 with your Railcard) whereas the LNER website is selling it for only £71.00.
Traditionally on many routes, particularly long distance ones, it has been the policy of those setting the fares for walkon (ie. non-Advance) Single tickets to cost not much less than a Return. In the case of the this flow from London to Edinburgh a Single is £1 (65p with a Railcard) cheaper than a Return.
It has been the policy of the various East Coast operators for a few years now to offer a Super Off-Peak Single ticket priced at half the price of a return when purchased in conjunction with an Advance in the other direction; the rationale being to encourage people to move to Advance tickets but where on one leg an Advance cannot be bought (eg. because Advance tickets are not on sale or perhaps the customer requires some flexibility on one leg). When Virgin Trains launched their new booking engine, it was not compatible with this requirement and could not issue this reduced-rate fare in conjunction with an Advance booking on the other leg. To get around this, the developers modified the booking engine and simply offers a ticket with 50% off, but this ticket is offered to all customers where the journey planner returns an itinerary on LNER trains (although it is still valid on permitted routes including those of other Tocs), not just to those buying an Advance in the other direction.
So accordingly, those who are only doing a one-way trip are actually benefitting as they are being offered a Single ticket at roughly half the price that it is supposed to be (Railcard customers are not getting such a great discount, but it is still much less than the price of the ticket would otherwise be even with the 34% Railcard discount). Railcard customers trying to buy this ticket as part of a two-way journey are however losing out.