It's random, with a 77% probability you won't have the option of getting what you need to research.
Seriously - I read - I think in a Tim Train or Chris Pine interview - that if two techs are needed for the next level, and you have one, if there are other same level techs available you will be offered these first.
e.g. you have, say,SoHB and want Cent Ecology to get to Cent Empathy. It will offer you any other level one tech you haven't got, before it looks at your research preferences. Then, say, you have Build/Conquer as your prefs, it will lead you thru the math/chemistry/loyalty level 2's before offering the level 1 Cent Ec.
I recall something to the effect that they had a "not greater than 10% diff" in the Explore/Discover/Build/Conquer bar graphs.
But that was over a year ago, in the old OWO forums. Any other oldtimers remember?
Googlie