When you click "Avoid Growth", the city planner automatically chooses tiles that gets the Growth total as close to 0 without going under. If you have very few low-food tiles, then you can still get positive growth. If the bucket is filled, the second feature of "Avoid Growth" comes in and prevents an additional population from spawning (and any additional food is lost). In other words, "Avoid Growth" tells the city planner to avoid food yields that make growth > 0 in addition to preventing additional population from spawning. To get the effect you are describing, the player would have to lock food tiles while "Avoid Growth" is checked.