Since disk platters surfaces are of course two-dimensional, areal density is a measure of the number of bits that can be stored in a unit of area. Of course the inner portion of the platter is where the spindle is, and the very outside of the platter can't be used either. Let's say about 1.2 inches of length along the radius is usable for storage.
Manufacturers usually quote the maximum linear density used on each drive.
Taking the product of these two values yields the drive's areal density, measured in bits per square inch. Also, it's generally easier to compute when the true areal density numbers are not easy to find. As long as they both have the same platter size , the comparison is valid. Otherwise you are comparing apples and oranges. The reason that density varies is because the lengths of the tracks increase as you move from the inside tracks to the outside tracks, so outer tracks can hold more data than inner ones.