There are two ways combat loses are determined after the battle. One is by direct observation: "I saw so and so killed." The other is by taking role: "So and so did not return." The text is silent as to how the number was determined, but implies it was done immediately:
So about three thousand men went up there from the people. And they fled before the men of Ai, and the men of Ai killed about thirty-six of their men and chased them before the gate as far as Shebarim and struck them at the descent. And the hearts of the people melted and became as water. Then Joshua tore his clothes and fell to the earth on his face before the ark of the Lord until the evening, he and the elders of Israel. And they put dust on their heads. (Joshua 7:4-6) [ESV]
Immediate reports of causalities are never precise. Some who were wounded could die later; some who were thought to have been killed may have been only wounded; some who did not return with the group may straggle in later. This is especially so in situations where the conflict involved pursuit. "Shebarim" (a hapax legomenon), may be "so called, perhaps, from the mines, or stone quarries, which lay in the neighbourhood"
1 or "crevices, or ravines. A short distance below Ai the road passes the head of steep glens, which open into the plain of Jordan."
Terrain with mines, quarries, crevices, or ravines offer the ability to avoid the pursuing enemy by hiding. So one who did not return with the main force is reported as missing, presumed dead, but may be alive.
The focus of the narrative is on the reaction to the defeat. It is easy to envision an exchange between Joshua and the leader of the returning company: "What happened?" "The men of Ai were too strong and we fled." "Did we lose anyone?" "About 36 were killed." Upon learning of the defeat and loss of life, Joshua tore his clothes and fell to the earth and was joined by the elders before the Ark.
There may well be some symbolic nature underlying the number; however, a report of "about" is how an initial report of casualties is made. That is to say, a good commander would report the number who did not return as lost in combat, hoping the actual loss of life is less.
- Cambridge Bible Commentary
- Ellicott's Commentary