Basically, the short answer is that most memory managers allocate more memory than a process needs, and then reuse it.

IE, in a JVM (Java) or dotnet (C#) process, the garbage collector allocates some memory from the operating system and keeps reusing it as it finds free memory and the program needs it.

These systems are built with the assumption that RAM is cheap and CPU cycles aren't, so they are highly optimized CPU-wise, but otherwise are RAM inefficient.