Why is there a difference in sound quality of old pianos?

I don't really know how to describe it, but there's a distinct overtone to a piano over a certain age-- or it just sounds like it. It's often portrayed that way as, for instance, a piano in a saloon-- whether it's just a Western trope to have them sound like that or there's an actual cause to it, I don't know.

My thoughts-- something due to a lack of maintenance, something in the wood itself, or something different in the manufacturing from that era. I don't actually know. Can anyone illuminate this? Because it's been kind of at the back of my mind ever since I noticed it.