Why does not the derailleur adjustment work the same way while riding? I suppose most of the people have noticed it. While adjusting both front and rear derailleurs, it will shift accordingly, when you have adjusted or are adjusting it, but when you test it by riding, the adjustment fail to perform the same way, Why does this happen? 

How does the added weight affect the shifters?
Frame flex under the added weight affects the relative position of derailleurs and shifters, which is reflected in the cable length. Movements of the cable when changing gears are on the millimeter scale, so even lengthening of the cable by a fraction of a mm due to frame flex can have an effect.