Prominence is hard to do automatically, so it may not be trivial to get the system that renders the osm.org tiles to prefer Philadelphia (it does things like look at population, but that data is present for Philadelphia).
I also took a look at it to figure out why it was showing up that way. They look like they are the same magnitude of thing (city) and they both have population. I'm wondering if it's actually showing Camden county?
No, it's not the county. I think what is happening is that the renderer decides there is no room to draw the Philadelphia label once the New Jersey label is in place.
