The calculated match is based strictly on the amount of DNA that you share. So the two of you share an amount that is typical of the 5th cousin relationship. You just didn't get that much common DNA from your common great-grandparents.
It is up to you to determine the actual degree of relationship in any match not ancestry.
I have a known third cousin whose probability of being a 5th to 8th cousin is rated as very low. We just didn't get much common DNA from our g-g-grandparents. I have a 4th cousin who is rated very high because we got a lot of common DNA.