I'll take a stab at this, though I'm certainly NOT an authority.

For every category (research, production, ship production), you have a base amount = some number. There are numerous factors that provide a percentage increase: base number x X% = the modified total. Percentage increases are adjustments of that base amount ONLY. They do not add a percentage of a modified base. That is, for instance, you have a base research value of 10. You build a Xeno Lab on a Research tile that adds 100%. The lab adds a 20% boost = a research total of 12. The tile benefit adds 100% of 10 = + 10. Then you build your Tech Capital which adds 100% of 10 = +10. What you end up with is 10 + 2 + 10 + 10 = 32. What you do NOT get is (10 + 2) x 2 x 2 = 48.

At least that would be my guess.

@CaptainPatch:
This is probably the explaination, it's just that i'm under the impression that it worked differently in DA. I never perceived the tech cap as such a disappointment when i built it. I rarely picked research boni before though.

Anyway thanks for the answers