This is almost certainly not because of the oil you used. I religiously tracked the oil use in my old 2001 Honda Civic, which I had purchased brand new. Every single tank was filled full and carefully logged. I changed to Mobil 1 oil/filters at about 10,000 miles in that car and I never noticed any significant improvement in fuel economy. None.
That said, what you're probably seeing is the result of two things:
1 - The car is breaking in. Cars tend to increase their fuel economy as they break in.
2 - It's obvious that you are concerned with fuel economy (your choice of car + logging each tank tells us that much!), so it's my hunch that you might be subconciously going easier on the gas pedal because you WANT or EXPECT to see an increase with Mobil 1. I'm not saying you'd do this on purpose, but it's a very real possibility.
A very slight increase is possible, although I personally never noticed one after 70,000+ miles of nothing but Mobil 1 oil/filters in my Civic.
Just my two cents.