Surprising increase in truck MPG

I have a Chevy Silverado pickup that is by now about 18 months old. 

I was never able to get more that about 220 miles out of a 22 gallon
tank of gas, sometimes even less. 

However, the last three refills, gave me 280, 266, and 283 miles per
refill. (I usually run until my tanks gets fully empty, since I have
an emergency gas canister for the instance when I run out of gas). 

This represents approximately 22% increase in MPG that I have hard
times explaining. I see three possible explanations.

1) Just before those refills, I did an oil change and used Mobil 1
synthetic oil. The previous oil changes were at a service station. 

2) The engine had a chance to "burn in" and naturally improved its
efficiency

3) something is seriously mistaken in my measurements. 

I am at a loss and am very puzzled. Can synthetic oil really account
for that much of an increase? I highly doubt it. 

i