The memory indicates how much RAM is currently used.
The memory under operational qualities indicates how much memory would be needed if all installed apps would use their maximum memory. This is calculated by the mem_limit defined in the app yaml file (and cannot be changed by the user).
This amount of data is the always allocated for this dedicated app and is the limiting factor. When the memory limit of the operational qualities is full no further apps can be installed although there is currently still unused memory left on the device, to ensure the apps will not run out of memory.