An image cannot consist of fractions of a pixel. Imagine you have an image that has a resolution of 3 ppi. That's 3 pixels within an inch of image. Now say you want a canvas size of 0.5 inches. That would mean an image consisting of 1.5 pixels, which is impossible. The image must be rounded to the nearest size that accomodates a whole pixel. You can either just decide that that's good enough (usually it is), or use a resolution so that your canvas size has more pixels that can be divided up into your desired size, then maybe you may be able to fit your resolution exactly into your canvas size.
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