You're right, of course. Sure, anything's possible. I know I have snagged an exit row with a generic A/B boarding before. Not often, but it has happened. And I have passed up the middle seat exit row many times. (which is often coincidentally taken by someone's laptop, brief bag, magazine/newspapers, ipads, etc. during boarding in hopes it will be ignored.)

I guess my point of reference was more personal. I have purchased a gate upgrade to BS before, on a long flight, I'm sleepy, knowing the flight is starting empty RON. And at boarding, I can make a bee line for the window exit row, stretch out and sleep. Then my chances are definitely increased if not guaranteed.