DVD drive splits into CD drive

This is a question of no major import. Just wondering.
Usually, as long as it hasn't been used, my DVD drive shows as just DVD
Drive D:\.
But once I use it for either a DVD or CD, it suddenly shows as 2 drives, D:\
and J:\. J being shown as a purely CD drive, while D remains a DVD drive.
Then as long as my system remains on, even if there are no disks in it, it
shows as the 2 drives in my system folder.
Why is this? Is there a way to get it to show as only one drive when a disk
is removed?
Just curious. Seems kinda weird, but that MS!
Thanks.