Auto-Lock Issue.

I had auto-lock set at 1 minute and I was listening to music on my iPod touch 5th Gen, the screen just dims itself but never lock or go into sleep mode. The iPod will lock itself after one minute anywhere else; home screen, safari, etc but if I am playing music or still in the music app it just went dim but never lock. I am running iOS 6.1.3. This only happens when I am charging my iPod.

Is anyone else iDevice the same? Will it lock itself when music is playing while charging?