PS does not make mathematically precise gradients. To every gradient PS creates it applies S curve. You can see it by historgam. As a result there are missed and doubled values along the gradient. Adobe thinks this is just what you want. Looks good, but for some adjustments it's not good at all.
One way to make clean black to white gradient:
Make Index color mode document, go to Image->Mode->Color Table dialog, choose Grayscale pallete and save it as a ACT file on disc. Find this file on disc and change it's extension to RAW.
Now open this file as raw image in PS. In Open dialog set Width - 256, Height - 1, Inteleaved - checked.
Now change the size of this image to 256x256 with Image->Image Size.
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