I think it's a bug. After doing a bit of experimenting and digging around, it seems that the price of a complément de parcours varies depending on whether you buy it from RATP or SNCF. (RATP runs the metro, buses, trams except T4, most of RER A and RER B south of Paris; SNCF runs all other suburban trains.) SNCF charges the expected price, while RATP charges more for some unknown reason.
In my experience, and yours, and other people's, SNCF charges the expected price: 5.70€ for a complément de parcous for a trip from zones 1–3 to Roissy CDG airport. This is the price of a ticket from Le Blanc-Mesnil (last station in zone 3) to CDG. I can confirm that this is the price whether buying the complément from CDG (which is an SNCF-only station) or from Paris at an SNCF ticket vending machine.
RATP charges more: 7.05€ (confirmed from several different metro stations). I have no idea where this price comes from. I found an official answer on a forum, stating that “the difference in price between [the] outward journey and the return journey is due to the airport tax”, but it makes no sense: the airport tax applies to both companies and in both directions.