That is how DOS always sets drive letters. From Wikipedia:
Assign the drive letter A: to the first floppy disk drive (drive 0), and B: to the second floppy disk drive (drive 1). If only one physical floppy is present, drive B: will be assigned to a phantom floppy drive mapped to the same physical drive and dynamically assigned to either A: or B: for easier floppy file operations. If no physical floppy drive is present, DOS 4.0 will assign both A: and B: to the non-existent drive, whereas DOS 5.0 and higher will invalidate these drive letters. If more than two physical floppy drives are present, DOS versions prior to 5.0 will assign subsequent drive letters, whereas DOS 5.0 and higher will remap these drives to higher drive letters at a later stage; see below.
Assign a drive letter to the first active primary partition recognized upon the first physical hard disk. DOS 5.0 and higher will ensure that it will become drive C:, so that the boot drive will either have drive A: or C:.
Assign subsequent drive letters to the first primary partition upon each successive physical hard disk drive (DOS versions prior to 5.0 will probe for only two physical harddisks, whereas DOS 5.0 and higher support eight physical harddisks).
Assign subsequent drive letters to every recognized logical partition present in the first extended partition, beginning with the first hard drive and proceeding through successive physical hard disk drives.
DOS 5.0 and higher: Assign drive letters to all remaining primary partitions, beginning with the first hard drive and proceeding through successive physical hard disk drives.
DOS 5.0 and higher: Assign drive letters to all physical floppy drives beyond the second physical floppy drive.
Assign subsequent drive letters to any block device drivers loaded in CONFIG.SYS via DEVICE statements, e.g. RAM disks.
Assign subsequent drive letters to any dynamically loaded drives via CONFIG.SYS INSTALL statements, in AUTOEXEC.BAT or later, i.e. additional optical disc drives (MSCDEX etc.), PCMCIA / PC Card drives, USB or Firewire drives, or network drives.
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