Most likely user error when they were assigning the car.
They likely didn't have any C's available, so they put you in a YB - but didn't tick the box to keep the rate the same so it re-calculated it based on a B rental (which is why it says "CAR CLASS Charged - B"), probably at todays rates rather than when you originally rented it.
(Or maybe they did it deliberately so as not to "overcharge" you for the C but didn't realize that the price actually went up from C->B)
One way or another it's clearly an error, and Hertz should be able to sort it out for you, potentially with a discount cert for a future rental to go along with re-charging you at the correct rate...