If the basement is unfinished, it also usually lacks a vapor barrier between the ground and concrete. Concrete will slowly wick some moisture into the space. But otherwise, that is correct, the dewpoints are pretty similar to the rest of the house, just cooler so the RH is higher. It is a common misconception that basement are particularly damp. I think that perception comes from how common foundation leaks are, poor humidity control in homes, lack of basement ventilation and leaks sill plates allowing outside humidity into a basement, and finally older homes often had stone foundations which actually allow some moisture to continously leech through the mortar. IT was just how they were and you kept the basement ventilated, only stored things in the middle and in winter it helped humidify the home and in summer it provided some evaporative cooling.