Individual process CPU and RAM use routinely due not add up to total use.

[HeyItsJono]

When my PC is sluggish I usually peak at System Informer and sort descending by either CPU or Private Bytes to check the culprit processes consuming all my CPU and RAM. What I often find though is that the sum usage of top processes is often as far as 20-40% off what 'total CPU' or RAM is being reported as. Why is this the case? I am running SI as an administrator.