Why top never shows ~100% CPU usage with heavy PCU load?

[Yuri]

When I run cycle process: main() {for (;;) {}} I never see that it 
consumes ~100% CPU.
Instead 'top -C' shows something like this, with numbers fluctuating 
around the shown numbers:


CPU: 96.2% user,  0.0% nice, 20.0% system,  0.0% interrupt,  0.0% idle
Mem: 653M Active, 995M Inact, 241M Wired, 90M Cache, 112M Buf, 11M Free
Swap: 16G Total, 204M Used, 16G Free, 1% Inuse, 16K In

  PID USERNAME    THR PRI NICE   SIZE    RES STATE    TIME    CPU COMMAND
85422 yuri          1  99    0  2520K   980K RUN      0:21 57.47% cycle
<skipped minor loads>


Total of CPU column for all processes is never what top shows in "CPU:" 
field.
At the same time kde system guard app shows 100% overall CPU load and 
same cycle app shows as consuming ~85% CPU.
So system guard always shows consistently higher number for the same 
process than top -C shows.

On Linux process the cycle app shows as consuming <~100% CPU and total 
is also <~100% that makes sense.

Why top's "CPU:" field doesn't equal total of all CPUs for all processes 
that it shows?
Why top doesn't show ~100% CPU consumption for the cycle process?
Why sysguard in kde consistently shows higher CPU consumption numbers 
than top -C?

Yuri