Poly does not add color, the stain used under it give you the color. Poly just seals it. Unless your using some of that not worth using Poly shades which has color added to it, but should never be used on a floor) Over time most poly will turn yellow or lighten up if there something blocking the suns rays like a rug.
One side could have been done with a stain called Natural or Maple, then sealed and the other side they just used poly no stain. If there was a rug over that lighter side the the old poly just yellowed (also called ambered)
Last edited by joecaption; 11-19-2011 at 04:08 PM.