Not to be a killjoy, but it makes sense.
Say you have a 757/767 reserve. But he's got to cover both international and domestic flying.
Most international flying starts about 4pm on the east coast and ends up in Europe at 8am.
So he's got to be up early enough to be able to cover 757/767 domestic flying, but then somehow catch a nap mid day just in case he's got to stay up all night and fly to Europe.
A little easier to split categories and have pilots that just do international thus staying current on Trans-Oceanic flying (folks, it aint just FMS and typing DIR/INTC CDG) and those that only do domestic and don't have to try to pre-plan getting called up at 3pm to fly all night to Europe without much warning after his rest period is up after covering a domestic trip.