Uneven CPU utilization distribution.

Hi,
My Cache has 8 CPU cores and config set accordingly:
$ cat /etc/unbound/unbound.conf | grep threads

number of threads to create. 1 disables threading.

num-threads: 8

Nevertheless 5-th core always has more than twice load than others:
1 [||||||||| 15.5%]
2 [|||||||||| 17.4%]
3 [|||||||||| 17.0%]
4 [||||||||||| 16.8%]
5 [||||||||||||||||||||||| 43.0%]
6 [|||||||| 11.4%]
7 [|||||||||| 16.3%]
8 [|||||||| 14.5%]

Is there any additional configuration stanzas or this normal behavior?