Yes, you fail to explain why a team has good and bad form over the course of a season, so I'll do it for you. From your perspective this needs to be simply random statistical variance, so form comes and goes for no reason, and averages itself out over the season. However, footballers are not quantum mechanical systems governed by statistics. They are classical systems governed by physical mechanisms. Form comes and goes for physical reasons. Not an intangible statistical reason. And a manager influences those reasons one way or the other by his physical actions and choices.
Same thing, from your perspective you need that managerial influence to just be blind chance, which can be described statistically and averaged out over a season. However, good managers are not blind morons who make decisions based on nothing. You are underestimating the processing power of the brain. Until you have a physical model for football as opposed to a statistical one, then the brain will always win out because it's working with a plethora of high quality sensory information and memories, and you're working with a paucity of stupid numbers that vaguely correlate to some vague property.