Zabbix aggregate checks shows wrong statistics

Please, help with a Zabbix problem. I configured aggregate check, which should calculate the group load on outgoing eth0 for the whole group "Linux Servers" I have in Zabbix. Here is a screenshot of my settings: http://gyazo.com/8a70cd96491a3c48f3539ac9622aaad7

I waited the graph line and surprised, because values on the graph are not real. Here is another screenshot I have: http://gyazo.com/ae76d13934952c7f49d0fadf99a7668a Problem is in that we don't have such uplink channel, it's less. So I think I have a mistake of configuring.

I found this topic: https://stackoverflow.com/questions/7097897/zabbix-calculated-item-function-over-multiple-items - and understood that my Item string is fine, in theory.

Why are values not real? Maybe I need something to add, or should use another place to configure this total check.

Thanks.