Seed features are identified by a value of 1 in the seed field. One seed feature is needed to grow each cluster. The tool found fewer seed features than clusters specified in the Number of Clusters parameter. Consequently, the tool will select the additional seed features needed to make up the deficit.
If you don't want to see this warning, you can modify the seed field you provided so the number of seed features (values of 1) matches the Number of Clusters you specify.