Shopping and math should not go together

A liberal arts education brings much learning, I'm sure. But I'm sure I couldn't defend it after my stunning academic performance today at the clothing store.

Here are the actions preceding today's special moment:
1) Received a $25 gift certificate for Christmas
2) Used said gift certificate to buy a $160 dress
3) Found a $25 coupon for the store from which I bought the dress
4) Returned the dress so I could re-buy it with the coupon

I wanted to make sure my gift certificate wasn't lost in any way, so I pointed it out to the lady at the register. She said

"OK, I'll just credit your account the full $160 and then you re-buy the dress. Is that all right?"

I nodded, but my mind did not follow in any way at all. She seemed to catch on.

"I want to make sure you understand..." She then started underlining figures and circling things to show me how it worked. While I understood that $160 minus $25 is $135, for some reason I still did not follow how the whole math thing worked out. Shouldn't the $25 be taken off the new purchase as well? My dress should only be about $100, right?

Eventually I made an embarrassed "Oh, I see" face and everyone seemed to think I understood. It was probably a full five minutes before I realized why it made sense. I pretty much needed to see the money change hands in front of me.

I'm not sure if I'm getting smarter or stupider-er