Assuming your standard pay is 500; in August you had to take 5% off the 500 which means 25 is deducted from 500 and you were paid 475. The next month, you had to take 5% off the 500 which is your standard pay not on 475, that means you were paid 475 again.
Therefore, your paid did not decrease by 10% in two months because a decrease of 10% means you were paid 450 in the second month.
