cpu single core usage always 0

When I monitored the boxes which were installed FreeBSD 7.2/8.0/8.1 distributions, I met a issue on CPU single core usage by using zabbix.
1.Using zabbix_agent from version 1.85 to 1.89
The key of "system.cpu.util[1,idle]" & "system.cpu.util[1,user ]" by using zabbix_agent (v 1.85 -1.89) catching data , the data of single CPU performance returned is always 0.
2.Using zabbix_agent before version 1.85
The data of single CPU performance returned is always the same. Compared with "Top" command returned info, it exist great disparity.