Jamieson, Fausset and Brown on 1 Chronicles 7:2

2. whose number was in the days of David two and twenty thousand and six hundred--Although a census was taken in the reign of David by order of that monarch, it is not certain that the sacred historian had it in mind, since we find here the tribe of Benjamin enumerated [1Ch 7:6-12], which was not taken in David's time; and there are other points of dissimilarity.