Our engine optimizes bids and keywords based on aggregate performance for a product's campaigns. As such, there may be a large performance variation for any individual keyword or campaign (manual/auto/PAT).
Put another way, some keywords will be below our ACoS target, some will be above, but the engine will work to ensure that aggregate performance is close to your goal.
If all keywords were below your goal, we would achieve better ACoS (as some keywords would never scale beyond a certain point), but this would likely be at the cost of sales volume.
If improving aggregate ACoS is your objective and you are fine with a decline in sales, we recommend making a change to your budgets and ACoS targets in-app.