Traditionally, the 6xxx range of NLCs was allocated to the Eastern Region, so both stations are "out of zone" as it were.
Conon Bridge, being in the Highlands, should have a number in the 8xxx range, and Stratford Parkway should have a number in the 4xxx range, being an ex-GWR route.
However, the expansion of internet sales and expansion of TVM installation has used up a lot of NLCs, and others are "reserved" for continental ticketing.
The obvious NLCs for Stratford Parkway, for example, would be 4557 or 4559. I can find no use of the former, but the latter is allocated to The Hague! Similarly, to be in line with numbering in that part of the world, Conon Bridge should take 8622 or 8623, with 8624 being allocated to Culloden.