The depth data is displayed as it comes from the sensor. So, check if your sounder is applying an offset on the display.
You can connect to your XB using our vmAIS PC utility and observe the NMEA sentences under the 'Serial Data' tab of vmAIS. The 'DPT' sentence will have the depth data
The depth is 3.4 meters in the above example
You can download vmAIS utility from www.vespermarine.com/xb in the downloads section. The utility is available for both Windows and Mac. After you download and install, you can establish connection to your XB either through USB serial or WiFi.