Zmokamok :
because Windows learns over time how to get the best use out of your RAM.
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What do you mean by this ^ ? First time I heard of Windows learning how to use the RAM more efficiently on it's own. Programs may go into a low usage mode and use less RAM after the computer was on for a bit, like utilities that check for updates. They run at start, use more RAM then once they do a check, the RAM usage by them drops.
To check RAM usage, just open up task manager, sort by RAM and you can see what's using it. If you have no issues, you could just have a bunch of programs starting up in the background, if you DO have issues, run a virus/spyware scan.