You're correct on that. Nordic had to pull their games from the catalogue a few years ago due to contractual obligations elsewhere, but were able to return the games during a sale promotion only at one point, and later returned for good. While we never knew the specific details of this behind the scenes (and quite frankly it's none of our business <grin>), a reasonable hypothesis is that Nordic had contractually binding terms for distribution elsewhere that had clauses concerning not giving other distributors any better pricing or similar, and that with GOG's own pricing changes it would require Nordic to accomodate a different pricing model that would have put them at odds to a contract elsewhere (Steam likely, pure guess). Taking it from there, it's likely that the contract they were bound to elsewhere didn't have any strict conditions concerning promotional sales elsewhere - which in theory would have allowed them to return their games here for the promotion without coming into conflict with contractual obligations elsewhere. Presumably after a certain period of time their contract expired and they renewed it under different terms that did not conflict with being able to sell games at GOG under terms that worked for both companies without conflict.

It's all speculative based on what very little information we do know that was given out by Nordic and GOG a few years ago, but there aren't really a lot of other ways to interpret what happened without going into unlikely dire conspiracy theories IMHO, so I believe this is most likely the actual case of what happened. :) Still - it is pure speculation.