The focal length is measured from the focal plane to the "optical center" of the lens. Using multiple elements and groups, and using a variety of types of glasses makes the center shift. One of the early lens innovations was the discovery that mixing crown glass elements and flint glass elements in lenses produced better images. The classic tessar design does owe some of it's success to it's triplet design, but it owes a lot to its implementation of mixing glass types as well.