Ach, sometimes things get lost in translation. RER/Metro is an important distinction (at a minimum, you need to remember to have your ticket ready for exit on the RER...) Perhaps the distinction is reinforced in Paris where the Metro was deliberately built to be incompatible with the mainline rail networks (narrow loading and track gauge, right-hand running) to avoid a scheme where all the mainline termini were to be destroyed at the lines interconnected underground as an alternative to the metro. There is also the slightly odd situation with RER line B (the main way of reaching both airport, unless you have a particular desire to use Pont de Rungis) where it is run by the SNCF (national rail authority) north of the Gare du Nord (so, to CDG) and by the RATP (regional transport authority, pronounced "rat-pee") south of it (so, to Antony for ORY).

Details, sure. But nobody wants you to get lost...