BTUs, or British Thermal Units, measure the heat of combustion. Even among the same species, different chunks of firewood are going to burn hotter than others, depending on how dry they are and the conditions under which they grew. These numbers are intended as a rough comparison.
It’s good to know that moisture content is more important than tree species when planning for wood heat. Wood that has dried properly will almost always burn hotter than wood that still has moisture in it, because much of the heat energy is used to evaporate the remaining water. Try to stack your firewood about a year in advance so that it has time to completely dry. You’ll get more heat out of your investment if you do.