ADXL345: How to get correct values

Hi,
I have cheap ebay ADXL345 Digital Accelerometer. I use simple code to check it:

#include <Wire.h>

#include <Adafruit_Sensor.h>

#include <Adafruit_ADXL345_U.h>

Adafruit_ADXL345_Unified BSens = Adafruit_ADXL345_Unified(1);


void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  BSens.begin();
}

void loop() {
  // put your main code here, to run repeatedly:
  sensors_event_t event;
  BSens.getEvent(&event);
  Serial.print("X: ");
  Serial.println(event.acceleration.x);
  Serial.print("Y: ");
  Serial.println(event.acceleration.y);
  Serial.print("Z: ");
  Serial.println(event.acceleration.z);
  Serial.println("|-------------|");
  delay(500);
}

And output is:

X: 2.55
Y: -14.32
Z: 20.04
|-------------|
X: 2.00
Y: -19.97
Z: 20.04
|-------------|
X: -1.57
Y: -14.47
Z: 20.04
|-------------|
X: 1.33
Y: -8.47
Z: 20.04
|-------------|
X: 7.14
Y: -11.10
Z: 20.04
|-------------|
X: -1.77
Y: -18.99
Z: 20.04
|-------------|
X: -5.81
Y: -20.08
Z: 20.04
|-------------|
X: -10.40
Y: -18.59
Z: 20.04
|-------------|
X: -13.85
Y: -11.53
Z: 20.04
|-------------|
X: -6.35
Y: -7.69
Z: 20.04
|-------------|
X: -1.80
Y: -8.00
Z: 20.04
|-------------|
X: 7.34
Y: -12.87
Z: 20.04
|-------------|
X: -1.10
Y: -20.08
Z: 20.04
|-------------|
X: -5.22
Y: -20.08
Z: 20.04
|-------------|
X: -3.14
Y: -20.08
Z: 20.04
|-------------|
X: -3.92
Y: -20.08
Z: 20.04
|-------------|
X: -3.92
Y: -20.08
Z: 20.04
|-------------|
X: -3.88
Y: -20.08
Z: 20.04
|-------------|
X: -3.92
Y: -20.08
Z: 20.04
|-------------|
X: -3.96
Y: -20.08
Z: 20.04
|-------------|
X: -3.88
Y: -20.08
Z: 20.04
|-------------|
X: -3.92
Y: -20.08
Z: 20.04
|-------------|
X: -3.92
Y: -20.08
Z: 20.04
|-------------|
X: -3.92
Y: -20.08
Z: 20.04
|-------------|
X: -3.88
Y: -20.08
Z: 20.04
|-------------|
X: -3.92
Y: -20.08
Z: 20.04
|-------------|
X: -3.92
Y: -20.08
Z: 20.04
|-------------|
X: -3.92
Y: -20.08
Z: 20.04
|-------------|
X: -3.92
Y: -20.08
Z: 20.04
|-------------|
X: -3.88
Y: -20.08
Z: 20.04
|-------------|
X: -3.88
Y: -20.08
Z: 20.04
|-------------|
X: -3.92
Y: -20.08
Z: 20.04
|-------------|
X: -3.92
Y: -20.08
Z: 20.04
|-------------|
X: -3.84
Y: -20.08
Z: 20.04
|-------------|

Z axis does not change!

Why?

Thanks!