The manual mentions a “Best Line” that should be flushed with the soil, which implies what @thrawed saw in the Amazon page. I’ve also seen people inserting it all the way until the plastic part is flush with the soil (which makes more sense to me).
I wonder if this is translation issue? Maybe the “Best Line” is the minimum the sensor should be inserted into the soil, and if it goes deeper, it’ll be fine. But it could also be the maximum, and inserting it pass that line could affect readings.
I really wish the manual would be more explicit.