Some folks , oddly enough do not select their seats in advance..even if not paying for selection.
I was speaking to an AC agent on the phone some weeks ago about a particular upcoming flight...I had a question about the seating as the Equipment had changed...resulting in a newly configured J cabin The seat map showed 10 "Open" seats out of the total of 24 in the cabin..on my screen...but she said "NO, there were only 6 open seats available" I told her that I found that curious..to say the least. She replied ..that it happens all the time ..that not all folks select their seats in advance..even in "J".
Go figure.
To comment on the OP's query about fare fluctuations... happens constantly..often without rhyme or reason. You have to grab the price at the time or risk losing it. Some routes will fluctuate up and down...but sometimes the fare will just continue to rise until sold out.