Hi,
Well, the quota bases itself on the "number of times used".
So if you change the "number of times used" in the coupon in the middle, you'll potentially be able to use it more.
For example, if I create aa coupon with a quota of 5, and there are 3 clients purchasing with it. The "number of times used" goes from 0 to 3. If I edit the coupon and change its value to 2, 3 more clients will be able to use it, for a total of 6 orders, instead of 5.
This could happen if you edit a coupon and someone places an order while you're editing the coupon. When you save the coupon, you might have saved it with the value from before the purchase, negating the increment which was done to the "number of times used" in the background.