If I had to guess, then the decision to go with insomnia deals was made on the fly during this summer sale, probably to boost volumes sold and break the ice so to speak. Those 24-72h deals on the front page respected the rule iirc, where they were not discounted prior to their highlight on front page and would go on a lesser discount once the time ran out.
I would imagine Insomnia deals would behave exactly the same if they were part of the strategy/sale from the beginning.
At least during the latter iterations of insomnia sales they did it that way, so they progressed from initial no-discount, to blitz deep-discount, to ordinary sale discount for the rest of the event.
Btw if the price difference with Solasta is considerable, maybe you could consider contacting the support. I'm sure GOG haven't done this to alienate anyone and I for one do hope that these insomnia deals won't be gone for another 6 years or how long it has been.