Welcome to the world of the GT/CS. They have their own minds and they will tell you how things will be from now on.
Telling you varying mileage amounts is it's way to get you to pay attention. No doubt, other electronics displays will have inconsistent information, which is another way of communicating with you----you'll see, just give it time.
You may wonder, is this like "Christine", "The CAR" or "My Mother The Car"?...well, no, but from now on, you'll always have this symbiosis with your GT/CS.
It now owns you.
Good thing you noticed this early on. Other '68 owners have been less lucky. Shooting cigarette lighters into the knee; wipers that work on their own; bouncing tach needles, etc.....and if you look closely at your '68 steering wheel, you'll see a smile (horn ring), eyes, (speed/tach gauges), and nose (pad center emblem). That was no mistake...no, sir.
You are now "one with the Special"....Resistance is Futile!
Paul "pulling yer chain" N.
(actuallly, your fuel sensor is so sensitive, that a cup more-or-less than a "fill up" of gas would give you varying mileage numbers like that.)