n_estimators总能提高RandomForest的性能？

Question

我获得了n_estimators最低值的最高分。根据我的理解，更多的树木应该总是提高性能。谁能解释一下这里发生了什么？

输入：

([mean: 0.87685, std: 0.03149, params: {u'n_estimators': 20},
  mean: 0.87551, std: 0.02979, params: {u'n_estimators': 50},
  mean: 0.87588, std: 0.02970, params: {u'n_estimators': 80},
  mean: 0.87545, std: 0.03043, params: {u'n_estimators': 110},
  mean: 0.87593, std: 0.02979, params: {u'n_estimators': 140},
  mean: 0.87506, std: 0.02913, params: {u'n_estimators': 170},
  mean: 0.87599, std: 0.02890, params: {u'n_estimators': 200},
  mean: 0.87559, std: 0.02875, params: {u'n_estimators': 230},
  mean: 0.87561, std: 0.02890, params: {u'n_estimators': 260},
  mean: 0.87500, std: 0.02867, params: {u'n_estimators': 290},
  mean: 0.87476, std: 0.02848, params: {u'n_estimators': 320},
  mean: 0.87434, std: 0.02800, params: {u'n_estimators': 350},
  mean: 0.87408, std: 0.02823, params: {u'n_estimators': 380},
  mean: 0.87461, std: 0.02789, params: {u'n_estimators': 410},
  mean: 0.87452, std: 0.02764, params: {u'n_estimators': 440},
  mean: 0.87466, std: 0.02775, params: {u'n_estimators': 470},
  mean: 0.87498, std: 0.02805, params: {u'n_estimators': 500},
  mean: 0.87530, std: 0.02797, params: {u'n_estimators': 530},
  mean: 0.87519, std: 0.02760, params: {u'n_estimators': 560},
  mean: 0.87498, std: 0.02789, params: {u'n_estimators': 590},
  mean: 0.87529, std: 0.02784, params: {u'n_estimators': 620},
  mean: 0.87526, std: 0.02792, params: {u'n_estimators': 650},
  mean: 0.87553, std: 0.02807, params: {u'n_estimators': 680},
  mean: 0.87540, std: 0.02794, params: {u'n_estimators': 710},
  mean: 0.87561, std: 0.02786, params: {u'n_estimators': 740},
  mean: 0.87554, std: 0.02814, params: {u'n_estimators': 770}],
 {u'n_estimators': 20},
 0.87684895838888188)

输出：

$message->body_str