Ok, say you are in a competition with a friend to see who can get the most weight in fruits. So you go to the supermarket, and you buy one grape. you take this grape and you are so excited, positive that you will win. You place the grape on the scale, then decide that you need more fruits. You run back to the store and buy a strawberry which weighs more than the grape. You come back to the scale and put the strawberry next to the grape. But, oh no, worms have eaten part of the grape, it now only weighs 95% of what it was before. Angered by this and concerned for your ability to win, you run back to the store and buy an apple. Upon arriving at the scale, you place the apple down, but the weight gain isn't as high as you expected. You look at the grape and strawberry and they both have been eaten by those pesky worms. Now the grape only weighs 90% of what it had originally, and the strawberry weighs 95% of what it originally did. Becoming more desperate to get weight, you run to the store for a fourth time, buying a pomegranate. When you get back to your scale, you find that the apple, grape, and strawberry have been eaten by the worms. The Apple weighs 95%, the strawberry 90%, and the grape 86%.
You continue this process until you have collected 200 fruits, and each time you get a new fruit, the worms eat the rest of the fruits. After your 200th fruit you take a look at the other fruits and notice: "Wow, the grape is so small, it's only 1% of its original size! o.o That means that its contribution to the total is very small. And then you realize further, that every time you bought a larger fruit, the worms ate the smaller fruits that you already had, so when you placed a 4kg fruit on the scale, the weight did not increase by 4kg, but by a bit less due to the other fruits being eaten."
Hope that clears up the weighting system a bit for you.