Albert Barnes Commentary Judges 20:15-17

"And the children of Benjamin were numbered on that day out of the cities twenty and six thousand men that drew sword, besides the inhabitants of Gibeah, who were numbered seven hundred chosen men. Among all this people there were seven hundred chosen men lefthanded; every one could sling stones at a hair-breadth, and not miss. And the men of Israel, besides Benjamin, were numbered four hundred thousand men that drew sword: all these were men of war." — Judges 20:15-17 (ASV)

Comparing the numbers here with those in Numbers 1 and Numbers 26, it is evident that in the case of both the Benjamites and the Israelites, their numbers were reduced by about one-third. That is, they amounted to only about two-thirds of what they had been at the last census in the plains of Moab.

This decrease seems to indicate disturbed and harassing times. With this, the mention of the cities as containing the entire Benjamite population agrees. The inference is that the open country and unwalled villages were not safe, and that the Benjamites kept the Canaanites in subjection only by living in fortified towns.

Regarding Judges 20:16 (see also Judges 3:15 and its note), in the Septuagint and Vulgate, the 700 chosen men of Gibeah are represented as the seven hundred left-handed slingers.