Why can we not determine the order of a reaction by taking into consideration the balanced chemical equation?

Explanation

The rate of reaction may not depend upon all the molecules of a reactant present in the balanced chemical equation. Thus, balanced chemical equation often leads to incorrect order or rate law.
`5Br^(-) (aq) + BrO_(3)^(-) (aq) + 6H^(+) (aq) to 3Br_(2) (aq) + 3H_(2) O (l)`
This is actually a fourth order reaction but it seems to be a twelfth order reaction. Actually, the reaction is complex and occurs in several steps. The order of such reaction is determined by the slowest step in the reaction mechanism.