Why does the transfers screen show that bytes are incoming for completed downloads?
Some files I'm seeding display hundreds of bytes incoming!
Thanks in advance.

Check bandwidth tab of that torrent. Most likely it's protocol traffic, for example other peer's requests and acknowledgments. You can set "Transfer B/s Display" to "File Only" in Settings > Output Formatting, if you don't like to see protocol traffic in this column.

I'm not sure I follow, what do you mean by 'protocol traffic' ?

And protocol traffic as opposed to what?

Protocol traffic is a service traffic. Opposed to it is a file traffic, a traffic of data.

In order to exchange data, bittorrent clients need to establish a connection between them. Clients use different messages to communicate with each other and keep connection alive. For example they use messages to: show which pieces of data are available, request block of data, inform that a piece is finished, pass IP addresses of peers, and so on. Even transferred data are also part of a message, which consists of a header, that identifies a block, and actual file data. Additionally a client needs to communicate with trackers or DHT to obtain IP addresses of peers.

This is why you see incoming bytes at transfers view, even for 100% complete, seeding torrents. Tixati has separate counters for protocol and file data, see Details tab of a torrent. Tixati shows protocol and file data in different colors, on the Bandwidth tab of each torrent. You can omit protocol counters in a few places by setting Output Formatting options to show File Only.