I've noticed this too. In my opinion, what happened was that we grafted the new word "zombie" onto a very old concept, which was "ghoul" - a graveyard monster that eats human flesh. The Haitian zombie ethnologists started to pay attention to in the 30s doesn't particularly resemble our modern zombie concept; it's a mezmerized/possessed living person who has to do the will of a sorcerer and is not able to control his/her actions. But the word was catchy, so we stole it and pasted it onto a monster we already had. You never hear "ghoul" anymore; it got pushed out by the new word. But it was all over the place in the 1800s.