Why does my relay write more bytes onto the network than it reads?

You're right, for the most part a byte into your Tor relay means a byte out, and vice versa. But there are a few exceptions:

If you open your DirPort, then Tor clients will ask you for a copy of the directory. The request they make (an HTTP GET) is quite small, and the response is sometimes quite large. This probably accounts for most of the difference between your "write" byte count and your "read" byte count.

Another minor exception shows up when you operate as an exit node, and you read a few bytes from an exit connection (for example, an instant messaging or ssh connection) and wrap it up into an entire 512 byte cell for transport through the Tor network.