Deal or No Deal

Here is the game... & I guess many people would have watched it. It is a sure craze from NBC channel at US.

My question is, what is the calculation behind it. In simple words, it seems like the banker's offer is some percentage of the expected value. So, all I did is played the game once to determine the expected values and the percentage offers made by the banker. Then I played once again and seems like the values tally. Here is how the calculation goes...

• When we have 20 boxes (after opening 6 boxes), offer is 10% of the expected value
• With 15 boxes left, offer is 30% of the expected value.
• With 11 boxes, it is 75%
• With 8 boxes, it is 83%
• With 6 boxes, it is 95%
• From this point onwards only one box will be opened before the next offer is made.
• We'll get 100% of the expected value from here...

So, now the question that stands is, why only those specific percentages are offered every time instead of directly offering the 100% of the expected value?