The number to the right of the weight in the log view is the difference between your measured weight and the trend line (moving average), not the previous day's weight.
FatWatch uses a moving average to smooth out your daily fluctuations in weight, and uses that to compute a trend line, which is the red line you see in your weight chart.
For example, let's say on Monday you weighed yourself for the first time and the scale read 285. Because it's the first measurement, we also start the trend line at 285.
The next day you weigh yourself and the scale says 255. To compute the trend for the next day, we take 10% of today's scale reading and add it to 90% of yesterday's trend value. In this case, that is (10% of 255) + (90% of 285) = (25.5) + (256.5) = 282. When you put that in FatWatch's log view, you see it like this:
This is why you see -27 (trend - scale) where you might expect to see 30 (yesterday's scale - today's scale).
If FatWatch only showed you the difference from the previous day's weight, you might see a lot of positive and negative numbers as your weight jumps up and down. By showing you the difference from the trend line, you get a better sense of whether you are generally gaining or losing weight.
For example, here is several days of weight, showing both the difference from the trend line and difference from the previous day:
|Scale||Trend||Difference from Previous||Difference from Trend|
As you can see, only showing the difference from the previous scale measurement hides the progress that is being made. From time to time your weight will go up, but if it is still below the trend line, then you are still making progress. By showing the difference from the trend line, FatWatch is letting you know that even if the scale says you're 3 pounds heavier than yesterday, you're still on the right track.