The cost of a rental car varies by car type, location, and dates. The car companies also have rates that fluctuate and can appear higher or lower closer to your arrival date, depending on how much traffic they are receiving. Some car types may not be fully booked, and they may show a lower rate closer to your travel dates, while other vehicles, such as the popular minivan can show a much higher rate as there may be few to choose from as they sell out fast.
There are different seasons for rental cars as well, and during prime seasons like summer, major holidays, and later December into January are considered higher season and prices are usually higher than low season rates.
Airport locations will have more taxes and fees related to the airport and the State of Hawaii, so their prices may end up higher than locations off-airport, but still servicing the airport with shuttles. As well, resort locations, although their taxes and fees may be lower, often have higher base rates to service customers in the resorts.