IRQ at 50% or maybe not?

[robk]

I have a FreeNAS System in the following setup:

Build FreeNAS-8.3.0-RELEASE-p1-x64 (r12825)
Platform Intel(R) Core(TM) i3-3220T CPU @ 2.80GHz
Memory 8052MB
System Time Thu Mar 21 15:15:50 CET 2013
Uptime 3:15PM up 37 days, 6:10, 0 users
Load Average 0.39, 0.32, 0.32

It runs smoothly with an Adaptec Raid Controller (4 HDDs)

The system-reporting graphs always show that CPU is constatly busy at 50% due to IRQ.
When I check the CPU usage on a terminal using 'top' I don't get any comparable values.

The picture shows the graph and the top output.

Is there actually something wrong with the system or is it a bug in freeNAS?
How can I check which reported CPU-usage is correct?

Thanks!