I would guess that it is on purpose, for the same reason that CO holds back EUA on the random empty DCA-IAH flights - the flight historically has shown potential to sell out in F, even if not on every flight or day of the week. My guess is that CO revenue management holds back EUA if a flight previously has any history of regularly selling out in F even if it is totally open two days before departure. As you say, the flight either sells out a week in advance or remains empty; therefore, the flight has some history of selling out in F and CO might want to hold back R seats in the hopes of it selling out again.