Because the % discounts cannot be added to calculate the final price.
The first discount is over the original price but the second discount is over the already discounted price.
You can find a combined factor if you multiply the two factors.
Discount of 25% => Factor = 0.75
Now you can find the combined factor by multilplying 0.75*.75 = 0.5625
That means that the final price will be the original price times 0.5625 (or what is the same that the discount is 100 - 56.25 = 43.75%.
Then the operation results in a higher price than if you multiply by 0.5 (50% discount).
In conclusion the discount resulting from two consecutive 25% discounts is less than a 50% discount.
