Total space 100
BTCApp: 30
ETHApp 2: 5
Coin3App 3: 50
Coin4App 4: 5
You tried to install the new BTCApp which is 40... so you uninstalled BTCApp... free space = 30 (can't install size 40, then uninstalled Coin4App... free space = 35 (still can't install size 40), then uninstalled Coin3App, suddenly free space = 85... you can install 40... and still have 45 space left!
Or it could simply be that apps need to be in "contiguous" space... so you can't split an app across multiple free slots and empty space needs to be next to each other? Maybe try and remove ALL the apps and reinstall the ones you want from most important to least important.
But, I honestly have no idea how the file system is setup... I can get 4 or 5 apps... and it's a matter of seconds to uninstall and reinstall an app if I need to change one.
Yes, your reasoning makes sense, at some point I will make some experiments to find out something more. I think I have deleted Neo, Stellar and Ripple to make spece for Bitcoin.