Why do charging rates differ for similar spec USB power adaptors? I've been experimenting with using USB power adaptors other than the one supplied with the Android tablet and I'm struggling to understand why adaptors of a similar spec to the original are charging at much lower rates.

Could anyone help shed some light on what's going on?

Tablet: Acer Iconia B1-750

Measurement App: Ampere charging meter app from Braintree https://goo.gl/SQNiQb

Adapter           |              |
Manufacturer      | Output Spec  | Charging Rate
================================================ 
Liteon              5.2V  1.35A   250mA        (original adaptor)
Delta Electronics   5.35V 2A      30mA
NewLink             5.0V  2.1A    20mA
NewLink             5.0V  1A      20mA



All measurements were taken using the same USB lead  
The adaptors were all plugged (successively) into the same wall
socket.  
The rate readings were taken after a minute or so of plugging in to allow the
charging rate to settle.
All the readings were taken within a few minutes, with the tablet battery level at about 50%
Each adaptor was unplugged and replugged a few times to check for consistency of readings.
All adaptors are single port


So, can anyone help me understand why the first three, similarly spec'd chargers aren't charging at the same rate?
User manual of your device does not offer any clue, but Acer community forums highlight the feature of original charger to do fast charging by delivering more current, which probably the other chargers don't have

From Acer Iconia won't charge, see reply of Acebillsey


  The charger that came with should have been a 5.35V 2A charger with internal settings that tell the tablet that it's a fast charger. When it's plugged in the tablet goes into fast charge mode and will charge even if you are using it. A standard USB port or USB charger will provide 0.5A and the tablet will only do the slow charge mode.


Again from Iconia One 7 (B1-750) not charging, same person says


  There are two styles of chargers. The stock one provides more juice than is allowed by the USB spec and they get around that by putting a special load between the data pins, so the tablet can see that the high power charger is installed and go into full charging mode. When that load isn't there the tablet will not draw more than spec (500mA) and typically the charge is just maintained


This fits in with your readings and probably is the reason, since there is no cause otherwise. Googling didn't help in getting fast charge details in original charger or OEM charging circuit to confirm this

Edit: Overview of USB Charging gives a high level view of modifications needed to force a charger to charge at higher rate