Grove Moisture Sensor - problem in reading

Good afternoon,

About my Grove Moisture Sensor (SKU: SEN92355P) I have 2 problems.

I’m using the code in the wiki: … or#Support
int sensorPin = A0; // select the input pin for the potentiometer
int sensorValue = 0; // variable to store the value coming from the sensor

void setup() {
// declare the ledPin as an OUTPUT:

void loop() {
// read the value from the sensor:
sensorValue = analogRead(sensorPin);
Serial.print("sensor = " );

I have this problems:
1º When the sensor is in the water I read values from 430 to 440, not over 900 as the wiki says. Why the difference?
2º Sometimes there is a inconsistence in the values and tend to increase, even if the sensor is still in the air. What coud cause that?

Thank you very much in advance.