Yes, that's correct. This is an unintended side effect of Avanti's "single leg pricing" initiative.
This involved Off-Peak Singles being cut to 70% of the cost of the equivalent Off-Peak Return, as opposed to the previous mixture of standalone Off-Peak Singles at ~£1 less, and Off-Peak Single (Online) tickets available for half the cost, but only as part of a return booking (i.e. in conjunction with an Advance or Anytime Single in the other direction).
As the "not via London" fare from Luton to Liverpool is, quite reasonably, valid via Sheffield (although it is clearly priced with the intention of use on the WCML), you can benefit from the "single leg pricing" policy if you are making a one-way journey.