Bit confused, how do you sell the individual game and soundtrack on sale separately but not the bundle?
Superliminal for all.

I don't like that as well, one example comes to mind is The Evil Within OST, it's not included into bundle and I think it was on sale only once since the release, for 50%:
https://www.gog.com/game/the_evil_within_soundtrack

That is by design ackshually.
It's a custom bundle for gog to simulate the 'bundle' that steam puts together.
The bundle on steam right now is 'on sale', 21% off for.. $14.98, which is the every day price on gog.

Developer commented about it here:
https://www.gog.com/forum/the_quest/about_deluxe_edition_price/post5

That bundle is always there at a better deal than regular price, for people to pick it up while there is no sales.
It is no different than buying The Quest + The Quest Soundtrack + The Quest - Islands of Ice and Fire.