I am looking at page 144+ in the Digimesh Dev Kit User Guide.
The devices passed certification with omni-directional antennas:
EU 15dbi @ 10dBm
FCC 9.3dB, no cable loss
So although the range might be based on a 2.1dBi antenna, the rules allow for a lot higher gain
For the EU, to quote this site:
...In Europe at 2.4GHz we are limited by ETSI regulations to 20dbmW.
That is 20dB gain above 1mW or 100mW Effective radiated power compared to an isotropic (0 gain) aerial. This figure includes the gain from the aerial and losses from cabling.
So to calculate the EIRP of your system:
EIRP=Output power of card or access point in dBm – cable loss in dB – connector losses in dB + aerial gain in dBi
A typical card gives out +15dBmW so the largest aerial allowed is +5dB. However on a typical installation an +8dB aerial can be used as there is quite often a 3dB loss on the aerial cable.
The exception to the above is France, where the maximum output power is limited to 7dBm (5mW EIRP) at 2.4GHz. ...
So for a Digimesh @10dbm, that should allow for a 10dBi antenna. The Digi manual says 9.3dBi so that is consistent. Same for the FCC.
So a more helpful answer would have been that the specified range of Digimesh modules is based on a 2.1dBi antenna.
The maximum antenna gain (zero cable loss) permitted in EU, Canada, USA and most countries is 9.3dBi.
That should allow for significant improvements in line-of-sight ranges.