wire crimper sizes: mm2 vs AWG
The reason for the discrepancy is that the AWG equivalent CSA you have found is solid wire. This type of wire carries the maximum current for CSA but also has the maximum bend radius. 

A tradeoff between current carrying capability for a lower bend radius is achieved by stranding the wire, i.e. 12AWG (2.1mm sq.) could actually be 37 x 28AWG and the new diameter (due to spacing between the 28AWG strands) is 2.9mm sq.

https://www.engineeringtoolbox.com/awg-wire-gauge-d_731.html

44 Spec Wire