It is because there is no such thing as a "1.44MB Floppy disk".
Floppy disk sizes were traditionally measured in KB, which was of course at the time, binary. so A "360K Diskette" was a disk that could hold 368,640 bytes.
The larger capacities for both sizes of disk included 1200KB diskettes (1,228,800 bytes) for the 5-1/4" diskettes and 1440K (1474560 bytes) for the 3-1/2" disks.
People started to shorten these and called them 1.2MB and 1.44MB. However 1,228,800 bytes is not 1.2MB by any definition, and 1,474,560 bytes is not 1.44MB.
Additionally, A formatted diskette is going to have additional space used by that. A Formatted 3-1/2" diskette will also have a boot sector (512 bytes), 18 sectors for the FAT and Mirror FAT, and 14 sectors for the Root Directory, which means an additional 16,898 bytes bringing the total usable space down to around 1.39MB.