Capacitive soil humidity sensor

Hi!

I have bought few of the "Gravity: Analog Capacitive Soil Moisture Sensor- Corrosion Resistant" and I am having a issue with the readings I am getting.
The values are going up when I water the plant, then decrease slowly and go up again when watering it each time. 
But you can see in the graph that freauently (once a day or every second day), the value increases a little and suddently for no apparent reason. I circled those in red in the picture above.

I am powering the sensor with 3.3V.
To save energy, I power those with the digital output pins only when I need the reading. 
Here is the procedure, each time I want to read the data:
1) Power the sensor with 3.3V
2) Wait 200ms so all electronic stabilize
3) Read the analogue value (I do 2 readinds and I dump the first one to be sure I have the right reading)
4) Power off the sensor
Between the raw analog input value, I use the "map" function to convert it to a purcentage, as it is in the graph.

Could you help me understand what is happening when the value increases for no apparent reason and how to solve it?

Thanks a lot!!