I would guess a significant part of the difference is warm-up time. When you fill up and the car's already warm, you're pretty much guaranteed to see higher FE than you normally would for the trip ahead--you're not going to be running the ICE nearly as much, and it must burn a fair amount of gas during warmup (the rest of the trip always starts out with recovering from the gas burned in the beginning 10 minutes or so for me). And with each of the other long trips, you have one warmup period for ~100 miles. At home you have one warmup period every _15_ miles, which means you're burning a lot more gas per mile.

At least that's how I'd interpret it. That first couple of trips is still pretty impressive though no denying. I don't think I've ever had a trip that high! My current tank (the last 2 weeks) is still currently hovering in the 45 range, highest I've had to date. Sadly this week I have family coming into town and my driving patterns are likely to change--not sure if I need to refill early just to lock in my mileage (lol) or keep going and hope I can still make my first 700 mile tank!