Another topic reminded me of this quirk:

My mercedes usually doesn't shut off when I have my doors closed. If I turn off the ignition, the engine will continue to run until I open the drivers door, and then it will slowly come to a stall.

If I open my door and count to five, the engine stops immediately when I shut off the ignition.

It doesn't bother me so much, rather I think it's an interesting quirk, but I can't help but be curious as to why it behaves like so.

Any ideas?