One word: Compass

Compass level drastically effects the battle once you start getting over 5 or so, especially when sending marginal numbers of ranged troops against foot soldiers. With their level 7 compass they'll be moving into melee range of your archers 70% faster than exercise will predict. And since the goal of an archer is to kill off all the attackers before they get in range, this drastically reduces the amount of time they have to accomplish this goal. Once a foot solder is within melee of an archer (especially swordsmen), it's pretty much over for the archers.

Moral of the story: Don't trust exercise. At best use it as a rough guideline but always double the number of troops it says will get the job done.

For reference, I wouldn't use less than 5000 archers against a level 7 valley with footsoldiers. It might be a tad overkill, but at least I know all those archers are coming home.