I'm struggling conceptually with the fact that I have a variable C that is the product of 2 other variables, A and B yet the annual change as a % in C is not the annual change % of A + B.
e.g
As you can see the total of the 2 is 13.1% not 13.5% - how do I explain this? Thanks Jacob
What if $A$ and $B$ both doubled? Then the percentages for $A$ and $B$ would be $100\%$. Would you expect $C$ to triple, i.e., increase by $200\%$? No, because $2\times 2=4$, so there would be a $300\%$ increase.
For very small percentages, adding like that is a good approximation. But what really happens when you increase by $8.9\%$ is multiplication by $1.089$. Similarly, increase by $4.3\%$ corresponds to multiplication by $1.043$. Combining both operations yields multiplication by $(1.089)\times(1.043) =1.135827$, corresponding to an increase of about $13.6\%$.
In general, $(1+a)(1+b) = 1+a+b+ab$. When $ab$ is small, which happens when both $a$ and $b$ are small, adding $a$ and $b$ gives a good approximation.