|I find this whole thread somewhat irrelevant - who uses FAT nowadays? But if we are going to criticize it, let's get it right. The OP is correct to say that the size of the File Allocation Table varies depending upon the size of the hard disk. It would be a pretty stupid file system (well, it is, but let's overlook that) if it relied just on the rather coarse-grained factor of cluster size. Cluster size is more important when determining the maximum size of a partition; double the cluster size and you double the number of sectors - and hence the size of disk - that a FAT entry can represent. Use that same cluster size with a smaller disk and you can get away with a smaller FAT table; but you waste more space on the disk as files must always occupy an integral number of clusters.|
There actually is a reason for that field in the boot sector that tells you how many sectors are occupied by the FAT - the reason is that it is not the same for all hard disks/partitions. If you don't believe me, try writing a program to access a FAT filesystem.
And, of course, it is also true that this metadata reduces the amount of storage available for data storage in any filesystem. So while the OP was wrong to ignore the 1000/1024 factor he was quite right to say that the metadata (not just the FAT) reduces the capacity; that largely accounts for the difference between riider's 74.48GB and the OP's 73GB.