Why use a different Internal File Name

I am running Windows 2000, v5.0 Service Pack 5.

In Windows Explorer, I selected folder %system%/system32/drivers, and sorted on Type.
I found a file named cdralw2k.bak. I right clicked on it and click on Properties. The description said, CDRAL for Windows 2000 kernel driver. I clicked on the Version tab and under Internal Name it said, CDRALW2K.SYS.

Why use file type bak on the filename that appears in the directory and internally use type sys?