Fellow was introduced to cater for the fact that formal education level in Germany is structurally higher than in anglosaxon markets (US, UK etc). Meaning that, traditionally, a BA is a position for Bachelors. However, in Germany the Bachelor's degree never had good recognition, and German companies would not accept a Bachelor as a Consultant who can be taken seriously (in fact, there wasn't even a really comparable university degree in Germany, because the first formal degree you would get from a German university would be a "Diplom", which is equivalent to the Master). So the German McK Office felt that they needed some intermediate entry position - more senior than a BA, but less senior than an Associate. This is how the "Fellow" position came to life.
Hope this helps!