The difference between the two contending armies is an even 20,000. I read this figure as symbolic, representing superior troop strength. And naturally, obtaining an accurate count of one’s enemy poses great practical difficulties. (Helaman, Part 1: Context, Chapter 4, “The Meaning of Numbers: Counts and Estimates in the Book of Mormon.”)
While Mormon is fighting with depleted forces, the Lamanite/Gadiantons are fighting with replenished forces. That last troop-strength numbers had been reported for the land of Joshua when the Nephites numbered 42,000 and the enemy 44,000 (Morm. 2:6–9). The trend is clear. The Lamanite/Gadiantons are getting more numerous, and the Nephites are getting fewer. Obviously, the Nephites have lost a greater proportion of men in rearguard fighting during their flight. The Lamanite/Gadiantons may have had proportional losses, but they are receiving reinforcements either from their homeland or from Nephite defectors (Morm. 2:8).