Two 5V2A powerbanks draw different current under same conditions?
The most likely reason is different charger requirements.

When you plug powerbank into charger, the powerbank needs to know max availiable current to prevent overloading it -- is it a regular PC port limited to 0.5A? Or 1A charger? Or 2A one? 

Unfortunately, there is no clear consensus on how to do it. USB 1.0 and 2.0 allow up to 0.5A current. USB 3.0 allows high currents, but requires (relatively) expensive chip to announce it. but various manufacturers came up with different ideas (short data pins, put different voltages on data pins, draw increasingly more current until voltage starts to drop, and so on).

Most likely, one of your powerbanks does not recognize your charger, and thinks it is an 1A model, and thus limits the max current. Since there is no standards, it is entirely possible.