I wonder if they're selling out or being consumed as upgrades? Domestically, I almost always buy an economy ticket on Delta and get upgraded to Comfort+ within 15 minutes of buying the ticket. That means the bus routes (between hubs) probably have a lot of these Skymiles high-tier members taking up the seats without charge.

That data, which I'm sure Delta has and looks at, could strongly influence how many overall rows of seating to remove in order to fit more rows of Comfort+.