Here's a thought -- HL has very few challenges anymore, the last big one was Amand_r's for Christmas, and then the remix, which wasn't stricly HL. So...two a year?
A symptom of a wave? What about stories not produced for a challenge?
The writers you mentioned have all been in the fandom for a while. I'd expect them to move back and forth through different concepts, able to skip around in the waves. (Hmmm, now I want to go swimming.)
But I think you're right about the first-time thing, mostly they don't focus on justification or explanation. Maybe they feel it has been done before, why rehash old stuff already written?