Xgboost: Early stopping with evaluation metric as AUC

I'm using the python version of Xgboost and trying to set early stopping on AUC as follows:

param = {
         'bst:max_depth':4,
         'bst:eta':0.1,  
         'silent':1,
         'objective':'binary:logistic'
         }

param['nthread'] = 10
param['eval_metric'] = "auc"
param['seed'] = 0

plst = param.items()

evallist  = [(dtrain_test1,'train'), (dtest2,'eval')]

num_round = 50
bst = xgb.train( plst, dtrain_test1, num_round, evallist, early_stopping_rounds = 5)

However, even though the AUC is still increasing, after 5 rounds the iteration stops:

Will train until eval error hasn't decreased in 5 rounds.
[0] train-auc:0.681576 eval-auc:0.672914
[1] train-auc:0.713940 eval-auc:0.705898
[2] train-auc:0.719168 eval-auc:0.710064
[3] train-auc:0.724578 eval-auc:0.713953
[4] train-auc:0.729903 eval-auc:0.718029
[5] train-auc:0.732958 eval-auc:0.719815
Stopping. Best iteration:
[0] train-auc:0.681576 eval-auc:0.672914

This looks to me somehow Xgboost thinks AUC should keep decreasing instead of increasing, otherwise the early stop will get triggered. Why is this the case and how to fix it?