For the conservation rate of power bank, I would like to explain it as below:

The store status of electric energy in power bank is chemical energy, the moving electric energy in USB cable is direct current. 

When we use power bank to charge a mobile phone, the energy conversion chart as below:

According to the conservation of energy,  in this small energy system, all the come in energy is from the power bank, and main energy consumption parts as below:

 

1. the internal electronic components in power bank ( e.g. when the power bank is discharging, it will be warm because of the resistance of components )

2. resistance of the USB cable 

3. the internal electronic components in mobile phone ( e.g. mobile phone is working during charging )

4. there will be some energy lost when it converts from one form to another form

 

When we see the energy in mobile phone, it can not be 100% equal to the energy in power bank, because we have to deduct the energy for all the consumption parts.

 

Please refer the conservation of energy from Wikipedia http://en.wikipedia.org/wiki/Conservation_of_energy