[argyllcms] Lab values in BtoA matrix

I am a bit confused about the Lab values in the BtoA LUTs. In my case, the BtoA LUTs are 33 x 33 x 33 and the abcissa for the points in the LUT are like: 

L  a  b
0  0  0
0  0  1
0  0  2
...
32 32 30
32 32 31
32 32 32
As I understand things for this LUT, an a or b value of 0 corresponds to -128 and an a or b value of 32 corresponds to 127.
My calculations then show that a value of 16 for a or b would not correspond to 0. Instead 16 corresponds to a value of -0.5. 

  16*255/32-128 = -0.5
I know this must be incorrect. Can someone please try to explain to me what I have missed. 


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