Research calculation on planet - possible bug

[Irrehaare]

Hi! I've been enjoying the game, but it stagnated for me due to low research and I've started mass investing into spice dens. Now I've noticed something weird and it's either me not understanding some mechanic, or a plain bug.

On a given planet (Thulun Prime) I have:
Starport 4 (2RP)
Palace 3 (2RP)
Exchange 4 (4RP)
8x Spice Den 3 (8*2RP)

This should sum up to 24 research, but the planet only generates 13RP, which is not even a possible sum from those factors.

Another planet has similar setup, but 4 spice dens, yet it generates 14RP instead of expected 16. I get similar story on other planets too. Am I missing a mechanic, or is it bug? I'm playing on hard, but I would expect some flat modifier applied, not such weird differing effects.

I'm playing on Hard, Tutorial map and on Android with Elite DLC.

[Dr. Spendlove]

What is the population on these planets?

[Griede Starless]

also, i think different difficulties effect the possible gains of buildings, but it has been a while since i played so i cant be sure.