Scale-up factor?

Hey I'm about to brew my first all-grain (2.5 gal) as soon as the materials come and so I was calculating out some sparge volumes for some recipes I'd like to brew. I was using this here calculator TastyBrew.com | Homebrewing Calculators | Sparge Calculator
It provided one number which I don't fully understand, scale up factor, and suggested that I increase my grain bill from 4.6 to 4.9 for batch sparging. What's going on?

I've never heard of that. It must be something in that software.

What are you making, and what's the recipe going to be?

I made a "fizzy yellow beer" today, to get ready for summer.

I'm thinking probably Biermuncher's Centennial Blonde, since I'm in Walla Walla Washington for the summer where it stays in the 100s most the time. Great wineries though. Besides the blonde ale, I'd also love to try Jamil's Mild or Southern English Brown. Really the main things I'm considering are low cost in case I mess up and a short time from grain to glass since I know I'll probably be impatient to try my first all-grain. In fact, I'll probably have to resist bringing an office chair down to watch the airlock. I also do half-batches, so the grain bills are kind of awkward (especially since Jamil gives recipes for 6 gallons going into the fermenter):
Centennial Blonde 4.38 lbs
Southern Brown 4.02 lbs
Mild 3.43 lbs
I'll be looking over my conversions again, since right now I was fevering and watching cartoons while formulating these recipes. Might want to double-check myself another day.

just a guess but it could be that the program you are using is saying that your efficiency will be lower. Basically they think you will get less sugar extracted from the grain than if you fly sparged.