This happens to me a lot. Even after 40K miles, I got 50.1 MPG last tank, 47.5 this tank. Same weather, same route. However over the long run, my mileage has gone up. It's not unusual to have "good tanks" - could be a combination of a lot of things. Enjoy it! Go another 4-5 tanks and see if it's a trend.
I have an Excel graph - can't figure out how to attach it right now. If I figure it out, I'll edit the post and add it.
Edit: Well, I can't figure it out. Here's some info on my 75 tanks for your entertainment.
Lifetime Avg: 45.47 (@ 40452 miles)
Best Tank: 50.09 (@ 39952 miles)
Worst Tank: 39.00 (@ 10626 miles)
Average variation in MPG between any two consecutive tanks: 1.84
Most variation in MPG from two consecutive tanks: 5.79 (@ 10626 miles - see worst tank)
Least variation in MPG from two consecutive tanks: 0.05
Statistically, 60-70% of my tanks will fall between 43.8 and 47.1 MPG
90-99% of the tanks will fall between 40.6 and 50.4 MPG
A lot of this variation is environmental (rain, sun/air conditioner, winter, wind, hills, traffic, etc.). I'm sure some is due to errors in the amount of gas that goes into the tank (I doubt the fuel pump "clicks off" at exactly the same fuel level each tank). Some honestly has no explanation - it's the magic and mystery of the hybrid.
Anyway, variation is expected. It's sort of sad that I know all the numbers.
Short answer is that your MPG should go up over time (car breaks in, improved driving habbits). Some will debate if the car breaks in at all, and the improvement is mostly due to learning how to get the most out of your hybrid. I think the car breaks in a bit, but driving habbits make the most difference.
Lastly, much has been discussed about gas. Most have concluded that octaine does not matter (go with what your car requires). I don't think a brand should make a difference - whatever minor difference it may
make would be burried in the noise caused by the environment.