Tax liens are a major derog. One will put you in a "dirty" scoring bucket. Multiple major derogs are each scored, however I would suspect that the algorithm weighs second and subsequent less. One of the mysteries under the cloak.
More significant is most likely the categorization, or "bucketing," of your file for scoring purposes. The scoring algorithm first places your overall credit file history into one of several different scoring categories, or "buckets," as part of its initial scoring. Weightings in actual scoring are different depending upon your scoring bucket.
One major derog will place you in a "dirty" bucket. Your categorization thereafter then does not become "clean" until all major derogs are gone.
Thus, the significance is that later deletion or exclusion of one tax lien wont show much score improvement, with the jump coming once the entire file is "clean."