No, it makes sense. Back in the medieval ages before PBS, companies had 'lines' consisting of trips and off days, and pre-built 'reserve lines' consisting of reserve days and off days and crewmembers selected their schedule from those. 'Line' is just a synonym for 'schedule', but typically used to mean 'a schedule consisting of trips and off days'.

Anyone else feel free to correct, but that's how we looked at it at my last employer (non-PBS), too.
(added: The (old) contractual definition of 'lineholder' includes reserves with reserve lines)