The Pigment molecules of semi-permanent colours are smaller than those of temporary colours and so are able to penetrate a little way into the cortex of the hair. The tend to wash out slowly and so last longer than temporary colours
Unevenly porous hair (e.g chemistry treated, permed, relaxed or bleached hair) will also take a semi-permanent colour unevenly. The cuticle scales are swollen and open in porous hair and the smaller molecules can penetrate deeper into the cortex in the porous part of hair, although they rinse off normally from the non-porous parts - producing patchy results.
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