Why are my Whatsapp stats wrong?

A while ago I was asked to prove who I had been sending Whatsapp messages to. Luckily my stats had been reset earlier that day. Unfortunately, my stats were showing that I had sent two messages, but when I looked at my chats from that day, I could only see one message having been sent. Can anyone explain this? In particular, is there any other action that Whatsapp might determine as a message sent?

This may seem trivial, but it's potentially very important.