Tech costs and times

[Bushface]

Having just completed Combustion, I wanted to research Flight, costing 7000 points, which showed that it would take 5 turns to complete. I happened to notice that I could instead research Fission, costing 7700 points, in only 4 turns. How come the more expensive tech could be done quicker ?
No, not because other civs already had Fission: my two most advanced rivals could both be offered Physics but had nothing reasonable to offer in exchange, even if I were willing to consider letting them catch up.
Can't remember if anything similar happened before Warlords, because I've usually made my mind up about what to research next and don't pay much attention to the alternatives.

 

[alexman]

Tech costs are affected by the number of prerequisites you have already researched.

 

[Bushface]

So what ? Fission has one pre-req - Electricity, which I got three turns before Combustion. Flight has two - Combustion and Physics, the latter being a pre-req for Electricity anyway. But how can the number of pre-reqs I have affect tech cost ? If I don't have the pre-reqs then I can't research the tech. I suppose that earlier on the tree this would be possible, when there are alternative routes, but not at this level.
Checking back on my notes, I see that I could have gone for Plastics (cost 9800) also in 5 turns - a more expensive tech, also with two pre-reqs which I had, in the same number of turns as Flight. The question remains: why should Flight be slowest to research ?

 

[alexman]

Have you already invested some beakers in Fission in a previous turn?

If you post a save, I could give you a definite answer. Otherwise we can only speculate.

 

[Bushface]

No, no previous research on Fission. And no, no save made at the time and the game is now over and deleted - but if I get something similar in the future I'll try to remember to make a save.