How mailgraph count message sent and receive ? I have two mail server postfix integrate with mailgraph and pflogsumm to monitoring traffic mail. ie: sample.com and example.com.

I have sent 1 message from sample.com to example.com and mailgraph at sample.com count 1 sent item and 1 receive item. But, why mailgraph at example.com just count 1 receive item even pflogsumm at example.com count 1 message receive and 1 message delivery. what does mailgraph works different with pflogsumm ? thanks for advance.