Indeed, the numbers are quite different. It has been suggested by some that Joab was negligent in reporting the numbers. It has also been suggested the difference in Judah is because the Benjamites are included in one account and excluded in the other. There is no hint of either solution in the texts.
Adam Clarke has done as good a job at addressing these differences as anyone I have read:
The former seems the most likely; but more corruptions have taken place in the numbers of the historical books of the Old Testament, than in any other part of the sacred records. To attempt to reconcile them in every part is lost labour; better at once to acknowledge what cannot be successfully denied, that although the original writers of the Old Testament wrote under the influence of the Divine Spirit, yet we are not told that the same influence descended on all copiers of their words, so as absolutely to prevent them from making mistakes. They might mistake, and they did mistake; but a careful collation of the different historical books serves to correct all essential errors of the scribes.
In any other book of antiquity, this would go unnoticed.
A response to 1001 Bible Contradictions.