JOKE - when you beat someone it gives you the rating you take them down to as your highest player yo

I'm a little confused to the fact when you beat someone, it gives you there rating you have took them down to as your highest win, surely it should go down as the rating you just beat the player @ I.E I have just know beat a player whos rating @ the time of play was 1316, when I checked to see the higest player i've beat, it was him but at the level I brought him down to 1306.... Can anyone share any light on why you don't get the rating you beat the player at as your best personal win and get the decline of rating instead?