Throughout the day as you eat, drink, breathe, sweat, and use the restroom your weight varies. In fact, from one day to the next your weight can vary by as much as 13 1/2 pounds.
That is why most people recommend that you don't weigh yourself every day. It is emotionally draining to look at the scale one day and see that you've lost 3 pounds, only to look at it the next day to see that you've gained 5 pounds. Most people, including me, can't take that kind of emotional abuse.
That was the genius of John Walker's The Hacker's Diet. He knew that those random variations are just noise. To really understand how you are doing on a diet you need to filter out that noise. And there are plenty of tools in the mathematical toolbox to filter out such noise. After a bit of trial and error John Walker settled on using an exponentially smoothed moving average. And that is what the SubtractTheFat uses too.
But there is one more way that we can add additional random noise to the system. Since we never eat the same thing at the same time of day. And we never do exactly the same activities at the same time every day. Our weight varies during the day in unpredictable ways. This is why I recommend that you weigh yourself every morning after using the restroom, but before eating or drinking anything. In my experience, this is the one time of day that all of the factors that affect your weight are most consistent.
Of course you aren't required to weigh yourself at the same time every day. The moving average that SubtractTheFat uses will smooth out the hour to hour variations the same way it smoothes out the day to day variations. But if you do keep your time of weightings consistent that is that much less random noise to worry about.