I have a question regarding the difference between stock and flow variables.
I know that a stock variable is measured at a specific point in time $t$ and the flow of this variable is measured over time, for example from $t-1$ to $t$.
When economists compute the ratio between a stock and a flow variable in some cases they use the geometric mean for the flow variable and the unchanged stock variable. Let's take an example: A very popular ratio nowadays is "credit-to-gdp". Nearly all studies compute the credit-to-gdp ratio for year $t$ by using the stock variable of credit at $t$ and the geometric mean of gdp from $t-1$ and $t$ (for example Mendoza & Terrones, 2008 or Dell'Ariccia et al., 2012). But why are they doing this? I know the difference between the two concepts but the question for me still remains "so what?".
Consider a counter example: income per capita. Income is obviously a flow variable while the number of people in an economy is a stock variable. But as far as I know nobody is even considering using the geometric mean or something like this.
Is it just a habit, a practical issue (if we have the data we do it if not we don't) or is there really a logical (mathematical) reason behind this approach? So far I was not able to find a convincing answer and I get the feeling that this is just a method to smooth the data a little bit.