The difference in perceived carbonation really hit home recently, when I was carbing a batch of Sahti. The recipe included a lot of rye, with a high mash temp as well. The beer was probably the heaviest-bodied beer I've brewed in about 200 batches. It seemed to take forever to carb, even at higher pressures. Finally, I determined that the heavy body was masking the carb level. Great beer, btw.