Strange capacitive soil moisture sensor readings
Your sensor is measuring the presence of water anywhere over its sensor area, not just at 20mm deep, from its general construction I should correct it to be more like a 40mm diameter cylinder about the sensing area, 

So if you have a single droplet of water in contact with the sensor it looks the same as half the sensor being near moist soil, 

I should also warn that bathroom silicone while preventing significant flow of water through itself, acts as quite a good humidity sponge, which again water near the sensor gives false readings, 

Air is a dielectric of 1, dry soil is 4-5, wet soil is 10-35, water is 80, so you need to correct any readings for the range you want when you do get it more reliable.