FAT was not based on the length field, it was based on the size of the index value. So, with floppies you had FAT12, where each cell in the FAT table was 12 bits, then FAT16, and then FAT32, which was 32 bits in size, BUT only USED 28 bits. exFat also uses 32 bit cell entries, but ALL 32 bits are used, and allows for
2^32 clusters with a maximum cluster size of 32GiB for a total file system of approx 128PiB
The file system was being called FAT64 because without knowing the structure of the file system, everyone just assumed it doubled the FAT from 32 to 64, and that is not the case. The creation of exFAT was to solve many problems. The 4gb barrier to file size was a biggie because a lot of video with high quality HD comes into play, where hours of video takes a lot of storage. Especially running high HD (1080P) and super high (4K) which can hit that barrier in 10-15 minutes. But the file system was also built for performance, especially for video recording. Higher write speeds, less overhead. For that, there is less FAT updates, the bitmap was introduced, contiguous file improvements, pre-allocated file spaces, to reduce overhead. exFAT is the standard file system for SDXC memory cards, because of the improvements in the the exFAT file system. One more thing: exFAT is also built to be flash memory compatible, less writes, block boundaries for better flash memory performance.
