Why does HTML Canvas getImageData() not return the exact same values that were just set?

Question

When writing pixels to an HTML Canvas context using putImageData I find that the pixel values are not exactly the same when I fetch them again. I have put up a

Answer 1

Looks like a rounding issue to me...

64/255 = 0.2509... (rounded down to give 0.25)
0.25 * 255 = 63.75 (rounded down to give 63)
== OR ==
64/255 = 0.2509... (rounded up to give 0.26)
0.26 * 255 = 66.3  (rounded up to give 67)

Remember that 255 is the maximum value, not 256 ;)

EDIT: Of course, this wouldn't explain why the alpha channel is behaving...