Why user time of the process depends on machine load?

[Yuri]

When I test performance of the code, I always observe dependency of CPU 
user time on the presence of other CPU intense processes.
Same CPU-only deterministic process that on the quiet machine completes 
in 220 user seconds in the presence of, for example, kde rebuild would 
complete in 261, 266 or even 379 user seconds.
I am talking about times shown by time(1), not actual an execution time. 
It's the same time as getrusage(2) returns in ru_utime field.

Why time that process takes in user seconds depends on what other 
processes are running?

FreeBSD-8.2 STABLE on i7 CPU @ 9200 @ 2.67GHz.

Yuri