entropy - I suspect that you had originally booked your direct flight with multiple segments (rule 1) and paid the extra $5 or so for your ticket.

The situation could be that there was 1R seat available for that segment - and someone in a higher fare bucket got EUA'ed. Then the next day, CO released one or more R seats for EUA the next day. I suspect that CO releases R seats gradually over the 5 day period - not all available on day 6 out.