Random songs, or not

I don't understand how my mp3 player works. It generally picks a bunch of songs and plays them over and over again, like a radio station but more portable. Then sometimes it'll grab something else that I haven't heard in so long that I wasn't aware I had a copy of it. Or as it's doing this weekend, a whole bunch of things I wasn't aware I had a copy of or hadn't heard in a long time.

Possibly it "favours" songs that I've manually selected to play and then does it random shuffle around them. I pulled out Grease the other day, when I was at my mother's, which I hadn't heard in a while, so that might now be the 'seed' that to jump to others from.

I wonder if it's worth tracking down the manual to see is there some deliberate logic behind it.