I'm pretty sure the ratio is calculated based on a formula to 'balance' the game based on items/gold available to PCs.
I bet Larian is looking at 10 items they put in every room and "weight-wise" it is kinda practical, to just slip 7 of them them in your pocket.
Then they add this calculation with every single item they put on the path and voila.
We must sell almost everything we actually never wanted to bring with us -> so that we can buy one article we want.
Feels annoying but it has to be done.
So that we can feel like we earned it or something.
Since "we worked so hard" for it by literally collecting all that trash.
