Is there a logic (or lack thereof) in how AC automatically assigns seat for those of us who do not wish to pay for pre-selection and/or AP miles?

I have recently consistently gotten crappy end-of-the-bus automatically-assigned seats that I change for much better seats towards the front at OLCI but I can't always do OLCI... Basically, it is clear the program does not by default assign the best available seat.

Any hints/advice/redirect would be appreciated for future convenience.