The Lamanites are now less organized than they were at the beginning. They have been attacked at the rear, and the rear has most likely caused some confusion in the center by more rapidly pressing their crossing that would have been in the original plan. As the army has come out of the river, they have again been pressed by fighting. As they continue to their objective, they may still have numbers, but they would not be as well formed into a combat unit. Thus when the meet the probably larger force protecting Manti, they would be at several disadvantages. They would be more tired, confused, and much less organized. This would give the Manti army a great advantage.
This suggests some more of Captain Moroni’s strategy. He has two types of troops with him. He has his own army that has marched up from Jershon. These are the ones that we know have the defensive armor. He also has an army raised from the land of Manti. That army might not have had the armor, and as a militia were probably not as well trained as the military more permanently under Moroni’s generalship. It would seem, then that Moroni used these tactics to fit the battle to the nature of the forces he had to deploy against the Lamanties. The “professional” warriors with the defensive armor would have been the two forces on either side of the river. As the unorganized and tired Lamanite army heads toward Manti, they would meet more opposing armies. This army would have been the militia. As less trained, and perhaps with less armor, they might not have been the equal of the Lamanite army is direct conflict, but they were not meeting the Lamanite army at its best, but one which would have had very disorganized battle lines at this time, and were already weary from both the fighting and the fleeing. Moroni has used his better forces to brunt the initial impact of the Lamanite army and soften them up so that his militia would be more effective.