Explaining it when you are telling your superior that no, in fact, you can't raise every kind of herb in the herbal at the castle, because the lands from which you fetch them range from a dry and sunny one to frosty and alpine, There's only so much you can do with planting them in the dark corners near the walls, or out where the sun shines but a wall protects them from the winds.
Especially when the castle itself is in a magical microclimate, up in the mountains. It's cool, all right, but all the architecture carefully structured to hoard the sunlight and ward off the wind would not keep it as warm as it is.
Indeed, knowing the castle, it may manufacture some microclimate herbals just to make a liar out of her. It's kinda prone to doing things like that.