When looking at the transaction history for a constituent or in a report, a transaction is listed as a registration fee although there wasn't a charge for registration for the event.
This is because the transaction is an Additional Gift.
For example, Peggy registered for the event at the same time as the transaction (this can be observed under her Interactions tab), and the donation was made as an additional gift during the registration process. Our system automatically classifies any money that comes through during the registration process as a type of "TeamRaiser Registration". Not because it is necessarily the registration fee, but because it occurred during that process. If she had registered, then come back and donated separately from the registration process, then it would show up as a "Donation".
One way you can tell if it is actually the fee, or an additional gift, is by checking the participant's registration page in Luminate Online. If it is the fee, it won't show up in the gift history. If it is an additional gift, it will appear. In addition, you can click on "refund" in the actions column on the transactions tab of the constituent record. In the popup window you can see the transaction breakdown.