Windows server memory - why doesn't it add up?

[ravenwitch]

Hi all, just a very quick question that is driving somewhat round the bend as noone can answer it for me.

I have been running some performance trending on a few of my servers and have notcied that the ram available and the ram in use do not always add up to the total ram in the box and they also do not tend to mirror each other i.e. as the ram in use goes down the amount of available ram does not increase.

Can anybody explain this somewhat illogical behaviour? Does it have anything to do with the paging?

Many thanks