The answer above says that radon is a heavy gas that tends to accumulate in low-lying areas. While radon is the heaviest gas, this has no significant effect on where it accumulates. It will quite easily travel to the top floor of high-rises.
The reason it is most often present at higher levels in basements, crawl spaces, cellars, etc. is simply that these areas are closest to the source of radon, i.e., the soil.