Maximum Virb Elite operating time

Did a little test today. Hooked up a battery charging source, with a fully charged battery and made a video to exceed the time of the charged battery. Did it; however when the battery operation, 'a single contiguous file done with 'time lapse', ends - then the green battery charging light comes one and further recording is done with single files created every minute of continued operation.

When the unit is connected to a battery charging source, why doesn't it operate in the same manner as it does when operating on the battery alone?