Don't know if this is enough for a primary response,

​

Other than the most obvious answer of incorrect records, most pre industrial battles had very lopsided casualties due to the nature of warfare. The goal is to break and rout your enemy, a fight to the last man would be prohibitively costly even for the victor, and such battles were rare.

In most cases, opposing armies would spend weeks harassing and maneuvering until either one was forced to battle by the other, or both felt confident of victory and engaged. As a result one side would often enjoy a massive advantage in position or suprise when they forced a battle.

Casualties during the main engagement would usually be proportionally low for both armies, most combat deaths came from the pursuit and destruction of a fleeing army. To further skew the numbers, wounded soldiers from the victor could be treated, while enemy captured and wounded could and would often be executed or die of wounds, exposure etc.