No you can't expect that.

It's about the efficiency.

No powerbank in this world has 100% efficiency. Even the charging circuit on your iPad can't be 100% efficient. There must be power loss here and there. Besides that, you have to factor in that the USB output is at 5V voltage while battery is normally at 3.7V and max out at 4.2V.

I came across a formula before, the actually capacity you can actually get on your device is just around 70% of the max capacity. Can be less. You really can't expect 5200mAh powerbank to give you 5200mAh out of 11600mAh battery in the iPad 4. Besides that, is your iPad powered on? I believe yes. That is another factor too.

Powerbank is not an unlimited source of electricity unlike the power socket in your house.
Wow... Seriously? That will be interesting!

Next time, switch off ipad and then charge it using a recently charged full 5.2K pb, then see how it goes. No point guessing esp when we don't know what is running with ipad on.