Prisoner Exchange - What Am I Missing?

In Alma 54, Ammoron, the leader of the Lamanite armies, sent an epistle to Moroni, the leader of the Nephite armies, requesting an exchange of prisoners of war. Moroni liked this idea:

And it came to pass that Moroni felt to rejoice exceedingly at this request, for he desired the provisions which were imparted for the support of the Lamanite prisoners for the support of his own people; and he also desired his own people for the strengthening of his army. (Alma 54:2)

So, Moroni sent a letter back to Ammoron, saying (among many other, less tactful things):

. . . I will close my epistle by telling you that I will not exchange prisoners, save it be on conditions that ye will deliver up a man and his wife and his children, for one prisoner; if this be the case that ye will do it, I will exchange. (Alma 54:11)

Ammoron wasn't pleased with Moroni's letter, but he agreed to Moroni's exchange terms anyway.

Nevertheless, I will grant to exchange prisoners according to your request, gladly, that I may preserve my food for my men of war; . . . (Alma 54:20)

Both sides wanted the exchange to happen. Both sides agreed to the same terms. This could very easily have been a successful and mutually-beneficial exchange of prisoners (and threats and insults).

But then Moroni called off the deal:

And he said: Behold, I will not exchange prisoners with Ammoron save he will withdraw his purpose, as I have stated in my epistle; for I will not grant unto him that he shall have any more power than what he hath got. (Alma 55:2)

But that wasn't what he had stated in his epistle. He had asked for one family for each prisoner he exchanged, which was what Ammoron had agreed to. It was a really good deal for each of them - a deal that Ammoron had initiated and Moroni had proposed. Moroni had "felt to rejoice exceedingly" at the idea of exchanging prisoners. Why did he call of the deal after Ammoron agreed to his original terms? There had been no haggling and no compromising. Moroni was getting what he had asked for -what he wanted. Why, at the last minute, did he call off the exchange?

One school of thought is that with all of the threatening and insulting going on between the two hard-headed war-captains, Moroni's pride got the better of him. He always did seem to have a short temper. Perhaps Moroni simply got fed up with Ammoron for stubbornly perpetuating the war they were in, despite how agreeable he was being about the exchange of prisoners. Perhaps Moroni was experiencing the same kind of doubts I started to have when I realized that anyone who suggests a deal thinks they're getting the better end of it. Perhaps Moroni suspected Ammoron of having some treacherous trick up his sleeve, though there's no mention of that in the scriptures.

Without a clear reason given in the text, Moroni called off what certainly seemed to be a deal that both sides wanted. Ammoron would no longer have so many mouths to feed, and Moroni would have secured the release of innocent women and children. Both sides would have traded prisoners they had to devote men to guarding for men who would fight for them. Maybe that was the problem. Maybe Moroni didn't want to release any Lamanite soldiers, even if he got one Nephite soldier for each of them, even though the Nephites had historically proven themselves to be twice as strong as the Lamanites, man to man, when the Lord was on their side.

This was a deal Moroni had proposed, and should have readily accepted. So why didn't he? What am I missing here? What reason for not exchanging did he see that I'm not seeing? If any of you know more about this story than I do, please fill me in, because from where I stand, Moroni's decision doesn't seem to make any sense.