Auto bracketing with self-timer (K-5)

When using auto bracketing with the self-timer set to 2 secs the camera takes the first shot after 2 seconds then inserts further 2 second timer delays between each subsequent bracket shot.

Now, after almost three years of owning a K-5/K-5IIs, I've just discovered that if the self-timer is set to 12 secs then the camera takes the first bracket shot after 12 seconds, then bursts off the reamaining shots.

I'm not sure why it does this. It doesn't seem to be documented in the manual. Perhaps the 2 second timer delay is intended to deal with mirror vibration whereas the 12 second delay is intended for bracketed selfies!