The Android OS does not work like a desktop operating system. On a desktop OS, like Windows, Mac OS X, or Ubuntu Linux, the user is responsible for closing the programs in order to keep a reasonable amount of memory available. On Android, this is not the case. The OS itself automatically removes programs from memory as memory is needed. The OS may also preload applications into memory which it thinks might soon be needed.

Having a lots of available empty memory is not a good thing. It takes the same amount of power to hold "nothing" in memory as it does to hold actual data. So, like every other operating system in use today, Android does its best to keep as much importantly/likely-to-be-used information in memory as possible.