Arduino Ground Current and DC power jack
Going by the schematic here there is an M7 diode (D1) between the power jack and the VIN pin on a Uno (you don't actually say which board you're using, I'm guessing based on the first link).
The datasheet here indicates that the maximum forward current for a M7 diode is 1.0 A. However different manufacturers datasheets seem to disagree as to whether that is a sustained DC current or only rectified current (so 50% load implying ~500mA constant load).
Either way you are looking at a potential 1.1 V drop across the diode so your VIN will be below the supply voltage by up to that much.

If you want to live dangerously you can always replace D1 with a piece of wire (or put the wire across it). You will lose reverse voltage protection on the DC jack, using a supply with it backwards will blow things up, but your current will then be limited by how much current the connector pins and PCB trace can cope with before they melt.