Why are Birkenstock size conversion charts imperfect?

European and American shoe sizing systems are like apples and oranges.

Most conversion charts make it look like the two "line up". Moving up one size in one system appears to be the same as moving up one size in the other. 

But that's not really right. American sizes are one-third of an inch apart while European sizes are two-thirds of a centimeter, about a quarter-inch, apart. A more accurate conversion chart would look something like this:

If that takes a minute to figure out, you'll appreciate why Birkenstock generally uses this much simpler, though somewhat less accurate, scheme: