It can be a result of bad fitting, where the optician doesn't measure your pupillary distance correctly, and doesn't line up the focal point properly with your pupil. Similarly, if you wear your glasses in a manner where it doesn't line up correctly (either because you like the way they look at a different spot on your face, incorrect frame adjustments, etc) you can get the same results.
(Not a subject matter expert, but my wife was an optician for 15+ years, so I've picked up a lot via osmosis)