Well, I got a prompt email from Steve at CatCon last night. He told me in 2004 Airstream wanted the monitor to show 100% charge at 13.75 volts, so CatCon programmed the units that way. He went on to say over the years Airstream asked CatCon to lower it to what is now 100% at 12.75 volts. He knew well that me and may others would never see the monitor at 100%, but as he said, that's the way Airstream wanted it at the time.
He did say they'd reprogram my unit free of charge, so I'm spending 5 bucks to mail it to Texas. He told me they'd ship it back same day. We'll see.
Easy enough to pop out - most wires plug in, but I had to snip 2 wires which I'll splice back together. 5 minute job.
Mystery solved, but I bet I'll have to recalibrate the tanks. Might need to pull a "cousin Eddie" calibrating the black water tank.