Alaska Airlines Mileage Plan - Seat Map under airport control, yet seats get taken?

Often I try and change my seat and I get the "unable to change seat" response. If I call the Alaska line, I get the "sorry, it's under airport control", but inevitably I will check later and the seat will be gone. Sometimes I'll call right back and have another agent try.

If it's under airport control, how does that happen?

Wirelessly posted (beckoa's PWP wondrousdevice3.0: Mozilla/5.0 (BlackBerry; U; BlackBerry 9810; en-US) AppleWebKit/534.11+ (KHTML, like Gecko) Version/7.0.0.261 Mobile Safari/534.11+)

Someone at the airport modified the seat- or possible changed through online checkin?

it wasn't online checkin yet...can an agent at the airport do this?

it wasn't online checkin yet...can an agent at the airport do this?Yes. In my experience, agents at the airport often proactively look for potential day-of-flight problems--MVP/Golds and MVP in lousy seats, families not seated together, that sort of thing--and try to fix them ahead of time.