This was the first enrollment made when Quirinius was governor of Syria.
Regarding some of the inscriptions mentioned by Robertson, these included those which were found in the autobiography of Augustus Caesar inscribed on the inner walls of the ruined temple of Augustus at Ankara.  These were published in the New York Times in 1929; and these refer to the two censuses, even giving the numbers of those enrolled and naming Quirinius in both as governor of Syria.  Luke is therefore quite accurate in his record.