Decision tree -only one object in the leaf

Hello, 

I've heard that when I have only object assigned to a class in a leaf, it means that my tree is overtested.

Could you tell me if this is something wrong with my optimize operator?

<?xml version="1.0" encoding="UTF-8"?><process version="8.1.003">
  <context>
    <input/>
    <output/>
    <macros/>
  </context>
  <operator activated="true" class="process" compatibility="8.1.003" expanded="true" name="Process">
    <process expanded="true">
      <operator activated="true" class="retrieve" compatibility="8.1.003" expanded="true" height="68" name="Retrieve bookratingsTagsUserNEW" width="90" x="179" y="85">
        <parameter key="repository_entry" value="bookratingsTagsUserNEW"/>
      </operator>
      <operator activated="true" class="concurrency:optimize_parameters_grid" compatibility="8.1.003" expanded="true" height="124" name="Optimize Parameters (Grid)" width="90" x="447" y="85">
        <list key="parameters">
          <parameter key="Performance.accuracy" value="true,false"/>
          <parameter key="Decision Tree.minimal_size_for_split" value="[0.0;0.0;0;linear]"/>
          <parameter key="Decision Tree.confidence" value="[1.0E-7;0.5;10;linear]"/>
        </list>
        <process expanded="true">
          <operator activated="true" class="split_data" compatibility="8.1.003" expanded="true" height="103" name="Split Data" width="90" x="45" y="238">
            <enumeration key="partitions">
              <parameter key="ratio" value="0.7"/>
              <parameter key="ratio" value="0.3"/>
            </enumeration>
          </operator>
          <operator activated="true" class="concurrency:parallel_decision_tree" compatibility="8.1.003" expanded="true" height="103" name="Decision Tree" width="90" x="179" y="85">
            <parameter key="maximal_depth" value="100"/>
            <parameter key="confidence" value="0.5"/>
            <parameter key="minimal_leaf_size" value="1"/>
            <parameter key="minimal_size_for_split" value="3"/>
          </operator>
          <operator activated="true" class="apply_model" compatibility="8.1.003" expanded="true" height="82" name="Apply Model" width="90" x="246" y="238">
            <list key="application_parameters"/>
          </operator>
          <operator activated="true" class="performance_classification" compatibility="8.1.003" expanded="true" height="82" name="Performance" width="90" x="380" y="187">
            <parameter key="main_criterion" value="accuracy"/>
            <list key="class_weights"/>
          </operator>
          <connect from_port="input 1" to_op="Split Data" to_port="example set"/>
          <connect from_op="Split Data" from_port="partition 1" to_op="Decision Tree" to_port="training set"/>
          <connect from_op="Split Data" from_port="partition 2" to_op="Apply Model" to_port="unlabelled data"/>
          <connect from_op="Decision Tree" from_port="model" to_op="Apply Model" to_port="model"/>
          <connect from_op="Apply Model" from_port="labelled data" to_op="Performance" to_port="labelled data"/>
          <connect from_op="Apply Model" from_port="model" to_port="model"/>
          <connect from_op="Performance" from_port="performance" to_port="performance"/>
          <portSpacing port="source_input 1" spacing="0"/>
          <portSpacing port="source_input 2" spacing="0"/>
          <portSpacing port="sink_performance" spacing="0"/>
          <portSpacing port="sink_model" spacing="0"/>
          <portSpacing port="sink_output 1" spacing="0"/>
        </process>
      </operator>
      <connect from_op="Retrieve bookratingsTagsUserNEW" from_port="output" to_op="Optimize Parameters (Grid)" to_port="input 1"/>
      <connect from_op="Optimize Parameters (Grid)" from_port="performance" to_port="result 2"/>
      <connect from_op="Optimize Parameters (Grid)" from_port="model" to_port="result 1"/>
      <portSpacing port="source_input 1" spacing="0"/>
      <portSpacing port="sink_result 1" spacing="0"/>
      <portSpacing port="sink_result 2" spacing="0"/>
      <portSpacing port="sink_result 3" spacing="0"/>
    </process>
  </operator>
</process>

For example something like that, where to the classses  I have one object assigned: