top -i doesn't show "heavy" jobs

Binary package hint: procps

I noticed this problem when opening a pdf document with evince, which put the load on cpu 1 to 100%, as seen from the system load applet. I ran 'top' (from the procps package) on the command line and saw the job there, too, with high cpu usage.
However, when toggling 'i' to only show me non-idle tasks, the process was not among the listed ones. This was consistently seen on various attempts, so not depending on time granularity. In non-idle mode I expect to see that process on first place in top's listing.

ubuntu 8.04
procps 1:3.2.7-5ubuntu3
kernel 2.6.24-19-generic