Yes, I agree with Cezary Kraszewski . The weight of fly ash concrete will be less than pure OPC due to the lighter density of fly ash. The higher strength of fly ash concrete can be attributed to the combined hydration and pozzolanic reaction of the binder system
As mentioned by others, PFA has a density of about 70% that of cement, so you would expect your cubes to be lighter. By 90 days the pozzolanic reaction should be sufficiently well-progressed such that the strength should be comparable or greater than that of OPC cubes.