Decimal difference in cost depending on order of Efficiency mods?

Yes, there's a rounding error.

It's a rounding error with the way mods are calculated. It's been in the game for longer than I can't remember. I don't think it'll be fixed.

Not 100% sure though, But i think warframe adds or substracts values to turn them to real numbers, So if the efficiency was > 169.5% and < 170.5% warframe just shows right away 170% to simplify numbers for players, Which explains why in the the Fleeting + Streamline swap it showed the same percentage but with red font.

About the energy costs changing with mod swaps, I think warframe does the math operations starting from the left to the right. In maths multiplying the efficiency with 2 percentages changes the value depending on which percentage is multiplied first, For example: Multiplying 50 by a 30% mod then multiplying the result by 40% and adding the result to the original 50 will come out with 76 (+26 than the original), However if you multiply 50 by the 40% first then the 30% it will result in 71 (+21 than the original). To sum it all up, I think the order of the mods affects the final result but by a small irrational number that is between 0.5-1 so warframe doesn't write it like that and rather shows it as a real number but still uses the original number in fights. So the Streamline + Fleeting swap is better.

Please don't take my comment serious as I am not sure at all if what I'm saying is true, It's just a theory that I thought of that somehow makes some sense.

In before "played 1,000,000 hours and this is the first time I realised this" posts.