Strange memory/cpu behavior

[Yuri]

I have STABLE-71 machine with 2GB memory and single 2GHz AMD3200 CPU.

There is one large active process slowly growing in memory from 500MB to 
1300MB, not reading or writing any files.
There are many dormant processes almost not running at all.
Swap size remains constant (185MB). Total physical memory used remains 
2GB (whole memory used).

ps shows that the active process only takes 15-18% CPU. But total CPU 
consumption on the machine is 100% (user).

Since the active process grows but swap+physical memory doesn't grow I 
assume that OS pushes out other processes code since it's unchanged on disk.

Why such operation is so expensive and takes 80-85% CPU?
Why total of all user processes CPU consumption is ~20% but total 
CPU(user) consumption shows as 100%? Shouldn't they be the same.

Yuri