Put in 4 gallons and still flashing pip and Add Fuel Message

I pulled into a gas station just as last pip starting blinking and "Add Fuel" message display on the MFD. I put in 2 gallons at first and the single pip still blinked. Went around the block came back and put in another 2 Gallons. I now should have 4 gallons + the supposed reserve that is stated in the manual when the warning pip begins to blink. I don't believe that there was 3 gallons reserve prior to filling with my additional 4 gallons but one must assume there was some fuel at that point. The outside temperature was 35 degrees. So at this point I'd assume that there must be my 4 gallons plus a minimum of 1 gallon reserve. Why the heck am I still seeing 1 pip blinking and getting the Add Fuel mesage with 5 gallons in my tank ? If you believe the manual, I'd have to had negative 1 gallons in my tank since adding 4 still triggers the warnings which are supposed to occur at 3 or less gallons ???

I also read that there are 2 warning messages - Add Fuel and Add Fuel Soon as posted previously. Is that true ? Add Fuel Soon seems to indicate more urgency - if so, how many gallons remaining triggers that message. All the manual says is that the blinking pip means 3 gallons or less remaining - I couldn't find anything about the MFD messages telling you to Add Fuel or Add Fuel Soon.

Now I know what you're thinking - why didn't I just fill it up instead of putting in 2 gallons and another 2 ? Well, I was on 1 solid pip and it was late at night. I thought I could let it go till next morning but when it started blinking I had only one station in proximity to get gas. It was a sunoco. This particular station has been the cause of my poorer than expected gas mileage. When I switched to a shell station (a bit further from my house), my mileage came back to where it should be. I've repeatedly noticed this problem from this sunoco gas but it took me 3 fillups to figure it out. So when I needed gas and the sunoco was the only game in town I figured let me just put 2 gallons in and get gas elsewhere. I then put another 2 in to satisfy the Add Fuel Message and flashing pip nag.

Any one have any ideas ?