Hallarmúli has similar composition as Hekla according to Haukur Jóhannsson and Kristján Sæmundsson (http://www.isor.is/sites/isor.is/files/Jar%C3%B0fr%C3%A6%C3%B0i%20og%20gossaga%20vestara%20gosbeltisins%20-%20Haukur%20J%C3%B3hannesson%20og%20Kristj%C3%A1n%20S%C3%A6mundsson.pdf)
Hallarmúli is measured to be about 6 million years old, and with the velocity of close to 2 cm a year, it should have drifted this far away from Hekla. It is also in accordance with the direction of the vectors.
This is a proof that the drift vectors, measured with GPS technology in 2004 compared with 1993 point locations, show real drift direction and speed. Actually, the proof is double, because a fault zone is accompanied with the sites of Hekla and Hallarmúli as shown below: