Why does my car has meter go down?

The gas meter or what're is called of my 2013 honda civic goes up and down i don't know why if it's only 7 months old is it normal? I took it the dealer they told me i have to take it when it happens but how thats impossible? What should i do

It usually happens when is in flat idle not turning or up or down hill

It will probably get worse with time just wait for a little while as it gets worse then take it in for warranty work