Don't forget that the 1 open duty period isn't going to stay the same all day. Yes 1 reserve will get called for that, but several other pilots could call in sick the day of requiring reserves, there could be a mechanical issue requiring a ferry or repo flight, or due to mechanical/wx/whatever a crew could be delayed and to minimize downline delays they may use a reserve crew to pick up their schedule. Lots and lots of different situations require them to call reserves outside of open duty periods shown on flica.