I keep coming back to this post trying to think of something that will answer your question.
I don't think I'm really sure what you're asking, though. If you are saying that if you log everything (food and activity), will FitDay calculate exactly your rate of loss, the answer is no. The main reason for that is because there are so many variables that can affect weight gain and loss, and FitDay (and most other programs) takes into account only age, size (what you enter as height and weight), and your estimate of activity level. These are all approximations at best, and when you add in measurement error in logging food, it is a ballpark estimate at best. Even estimates of what you burn are largely inaccurate, unless you wear a heart rate monitor, and even then it's no guarantee...it just goes by heart rate and whatever other data you've entered in.
Losing is not an exact science...it would be awesome if we could have a direct linear relationship between calories eaten and expended and weight loss, but it doesn't work that way. Metabolism is affected by proportion of muscle and fat, for instance, so even two people weighing 150 lbs. and eating 1400 calories a day will lose at a different rate if one is 20% body fat and one is 35%.
The only thing I can think of that would bring you as close as you can get to predicting your weight loss would be to have your body fat percentage done (usually hydrostatic weighing for most accurate results) and when you do that, they can pinpoint how many calories you burn more so than when you select "seated, some movement," etc. You mentioned "average, slow, and fast," but those are still very broad. And even with a scientific accurate measurement of metabolism, plateaus, water retention, sodium, time of month, stress, and a gazillion other factors can affect your rate of loss.
Bottom line, what FitDay gives is about what you will find for a "do it yourself" calculation, without going into some fancy health/exercise facility and getting measurements of everything done down to the nth degree. The software is good, but nothing is that smart to do it perfectly. It is an awesome program for support, though.
I hope this helped; if it didn't answer your question, maybe rephrase it and I or someone else can take a better stab.
"If you drop an egg, you don't say, 'Oh, shoot' and drop the other 11, do you?"
-Source unknown, but obviously brilliant
Reached goal 4/16/2010...but kind of afraid to look these days