Thanks for highlighting the issue, i also have a Honda city 2010 model.
I actually face actually the same problem, facts are when i fill the petrol tank for approx 10 or more liters, the needle shows the petrol available in my tank and whenever i disconnect and reconnect the battery, it shows the petrol
At times when i open the petrol cover, air blows out of tank and needle again starts showing petrol.
Your observation is matching with mine that it shows average of petrol even if we drive on CNG, although it goes upto 45kmpl but reduced the petrol accordingly and the Range of petrol is also misleading.
I havn't also been able to get the issue resolved, longing to get it resolved as i am always afraid of running out of petrol as it doesnt even start directly on CNG.