I don't know if this is the case for Seinfeld, but TV productions filmed in 16:9 but for 4:3 broadcast didn't always take care that the full 16:9 frame would have adequate content, i.e. no mike booms or non-stage items visible, and therefore later 16:9 versions had to be zoomed in by some factor; still wider than the 4:3, but reduced in the vertical.