How Many Disciples in Luke 10---70 or 72?

In Luke 10:1 (NAB), we have: >After this the Lord appointed seventy[-two]* others whom he sent ahead of him in pairs to every town and place he intended to visit. The gloss therein, indicated by the asterisk says: > Seventy[-two]: important representatives of the Alexandrian and Caesarean text types read “seventy,” while other important Alexandrian texts and Western readings have “seventy-two.” In the D-R (Luke 10:1), there are no brackets: > And after these things the Lord appointed also other seventy-two: and he sent them two and two before his face into every city and place whither he himself was to come. Can anyone explain the gloss a bit further, indicating how or why a difference in numbers appears between two versions of the Bibles? Undoubtedly, there is a *significance* to the exact number of disciples Our Lord sent out, but it will be difficult to infer if the exact number is not known.