scriptclip problems

[Wilbert]

I was trying scriptclip (on some almost black frames), and got the following output:

# gives 43.66
clip = clip.Trim(0,-1)
ScriptClip(clip, "Subtitle(String(AverageLuma(clip)))")

# first frame: 43.656
# second frame: 43.395
# third frame: 43.405
clip = clip.Trim(0,-3)
ScriptClip(clip, "Subtitle(String(AverageLuma(clip)))")

# gives 0.00002
clip = clip.Trim(0,-1)
ScriptClip(clip, "Subtitle(String(YDifferenceFromPrevious))")

# first frame: 2.271
# second frame: 0.00002
# third frame: 2.137
clip = clip.Trim(0,-3)
ScriptClip(clip, "Subtitle(String(YDifferenceFromPrevious))")

There are two things which I don't understand:
1) Mustn't the order in the last example be 0.00002, 2.271, 2.137 instead of 2.271, 0.00002, 2.137.
2) Even then, I don't understand these values. I also added the calculation of averageluma. The difference between the first two frames, for example, is 43.656 - 43.395 = 0.261. Which is not approx. equal to 2.271?