I'm not sure if this is true. I've done a lot of high speed (80-85mph) freeway driving on my car and it gets around 37-38mpg under these conditions. Filling the tank on these long trips (Bend,OR to Phoenix and back 3 times) gives me a DTE of around 475 and I know the tank is full (needle width above full mark).... if you're getting a DTE that's under 500 after you fill up, you didn't fill up all the way.
When I drive around my house I get around 45mpg and when filling up during these drives the DTE is low to mid 500's.
The DTE is in part determined by past MPG. High MPG history gives you higher DTE numbers (assuming the tank is always filled).