If that is the case, then they really need to alter the Information Index, which simply states that 'Improvements are stackable'.
Also, how do they then work out which multiplier comes first for those that have unequal effects?
Again with barracks and guerilla camps, one boosts soldier efficiency by 10% and one by 35%
So (assuming one of each) does it:
1) take the base figure and increase by 45%
2) take the base figure and increase by 10%, then increase that figure by 35%
3) take the base figure and increase by 35%, then increase that figure by 10%
Quickly checking the maths, assuming an efficiency of 100:
1) = 145
2) = 148.5
3) = 148.5
So it is irrelevant which way round 2) and 3) are... and I assume they don't use 1)?