Relying exclusively on the highest string similarity for the normalization, would lead to candidate lemmas that result from linguistically unmotivated operations, e.g., the German Hühner 'chickens' would be normalized to the most string-similar lemma Hüne 'giant' (ED=2) and not to the correct but less string-similar lemma Huhn 'chicken' (ED=3).
a linguistic restrictor is indispensable for finding the underlying lemma for a given constituent.