hah, it goes up and down. That free memory is not just installed apps, but also app cache too. The phone doesn't keep this value up to date every second, to do so it would have to constantly scan memory and thats cpu and power hungry process.
Thats why doing a reboot typically changes the value in a way that looks strange. Many things may have happened since the last time it fully refreshed, so its hard to keep track of how an uninstall will effect free memory for example