Why do charging rates differ for similar spec USB power adaptors?
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