Because it's not a cryptographic hash where a one bit difference results in a completely different hash. It's a perceptual hash that operates on a smaller bitmap derived from the image so it's plausible that some innocuous images might result in similar derivations; and there might be intentionally crafted innocently-looking images that result in an offensive derivative.
Salvador Dali could do something similar by hand in 1973 in Gala Contemplating the Mediterranean Sea [1]