Actually, they know exactly what they are doing, they just do it different then you. Many figure manufactures measure the figure from the base of the foot to the eyes. I can't tell you where most manufactures are today, but in the past almost all did it this way. This is where the 1/56 scale figures came from. Bolt Action Miniatures, Company B, Sloppy Jalopy and many others have been making vehicles in 1/56 scale for years.
If you want you can read more about scale in miniatures on sites like The Miniatures Page. They have some good articles in there features section that have been there for years.