Why are Birkenstock size conversions 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 true. American sizes are one-third of an inch apart, while European sizes are two-thirds of a centimeter, or about a quarter-inch, apart. An accurate conversion chart would look something like this:

The two systems do line up at Birkenstock 40 and US Women's 9. The farther away from that point, however, the less accurate standard charts will be. Birkenstock 36, usually converted as US Women's 5 to 5½, is really closer to US Women's 6.  Birkenstock 45, usually converted as US Men's 12 to 12½, is really closer to US Men's 11.

The challenge of providing useful conversions is further compounded by the footwear industry's lack of uniformity. Different brands have different ideas about sizing. Different models even within the same brand, in the same nominal size, often fit differently.