That would be pretty surprising, unless your bank charges you unusually high fees. I assume you paid an ATM fee to get the USD, then you got about 3% less than the official exchange rate at the cambio. My experience is that I get charged a fee on the order of $3. per transaction from the ATM regardless of what currency I get, and then get within 1% of the official rate on the withdrawal if it's in the local currency not USD.
Of course, if you're bank is Scotia, they may not charge you the ATM fee. But I've always gotten a better exchange rate from ATMs than from cambios. Not enough to make a big deal out of it certainly, but no advantage for me in doing what you suggest.