I would refer to the instructions, which will show how to set it up. It will not be accurate if it is not set up. My roommate gets 40+ mpg in her Escape until she gets to the fuel pump- yet never calibrates. It's frustrating for everyone.
This may also help:
calibrate - Google Search
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It's most definitely because it is not set up. Mine is always off in the Mustang because I EOC, but it is LESS than right. I often check it, knowing I will get better than that. There are settings for it, but I like to see how far I EOC. The easiest advice is to set it up correctly, and then keep correcting it at each fill up.