All SD Cards mount as Internal
What you are seeing is expected behavior and how all Macs with built-in SD card readers are treated by the OS.

If you want to split hairs it is indeed "internal" as the card itself is inserted into the Mac, even though a bit of it still stacks out. 

Regardless of that it is still expected behavior and not a problem. As to why? well only Apple hardware and software engineers can give you a definitive answer on that and they aren't allowed to say