Low nutrient farming...bug?

[OlivePit]

For thorough testing I purposefully drained a field of nutrients by planting wheat repeatedly on it.
At 39% nutrients and 150% fertility it allowed me to plant wheat, ok, cool.
But it harvested 411 grain which is significantly more than it should be. bug?

At the very least it would not let me plant again at 9% nutrients, so that is good.