Why is it impossible to simply add trade discounts together when calculating the net price by using a series of trade discounts?

Think of it this way,

$150 @ 20% net 10 days, 15% volume purchase, 15% sale.

that would be (in the total 45%)

so 150 X .45 would yield a due amount of $67.50 - - $150 - $67.50 = $82.50
However, if you step down the percentages the net will be different because the balance changes.

$150 X .2 = $30 - - $150. - $30. = $120.

$120 X .15 = $18 - - $120.- $18 = $102.

$102 X .15 = $15.30 - - $102- $15.30 = $86.70