Commentaries:
Barnes' Notes
The enrollment, being taken principally for military purposes (compare Numbers 1:3, Numbers 1:20), would naturally be arranged by hundreds, fifties, etc. (cf. II Kings 1:9, II Kings 1:11, II Kings 1:13). In eleven tribes the number enrolled consists of complete hundreds. The difference, in this respect, observable in the case of the tribe of Gad here Numbers 1:25, and of the tribe of Reuben at the later census Numbers 26:7, is probably to be accounted for by the pastoral, and consequently nomadic, habits of these tribes, which rendered it difficult to bring all their members together at once for a census. Judah already takes precedence of his brethren in point of numbers (compare Genesis 49:8 note), and Ephraim of Manasseh (compare Genesis 48:19-20).
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