Noticed that sometimes on larger enemies. Ogres for examples. This is just speculation, but it seems like the "5 ft" distance is calculated from the center of their hit box, and not the edge. So the bigger an enemy, the less likely it is that you will be able to stand within 5ft of the center of their hit box. Haven't had this problem on smaller enemy types like goblins. Again, all speculation and haven't thoroughly tested it.
