Something I learned a few trips ago and it does make sense.
I had issues using the total US amount on a Canadian Gift Card when paying my room charges before checking out, it would not go through at all and it was upsetting as it was a $500 Canadian Card. A fellow Canadian who happened to be at the resort counter next to us said, try the transaction with one penny less and it worked. He explained that when the conversion is done it rounds to the nearest penny. So sometimes the true conversion, if rounded up, is not quite that whole penny. That's why sometimes it works and sometimes not, depending on the going rate and how it was rounded up or down. Never had an issue since. Now when using the whole balance I just tell them to put it through at one penny less.
After, I just transfer that penny balance showing from each card I've used and combine them to one card on the Gift Card Balance site for next time.