[Q] Charging Question topic

I just received my Padfone X yesterday. So far, I think I made a good choice. Once it is rooted and all of the crap removed, it works well. I need someone to help clarify something for me...

Should my phone charge when in the tablet?

So far, what I am experiencing is that, if the tablet is plugged in, the tablet and phone charge. However, if the phone is in the tablet and the tablet doesn't have AC power, the phone battery doesn't charge. Not a huge deal but I would like to understand if that is by design or if I have a problem.

Thanks.