QUndoStack problem

[swiety]

I have a QTreeWidget with rows:

A
B
C
D

and commands:
1:

QUndoStack *stack = new QUndoStack();
stack->push( new InsertCommand( new QTreeWidgetItem(), 0 ) );
stack->undo(); // insertTopLevelItem( 0, newitem );

QUndoStack *stack = new QUndoStack();
stack->push( new InsertCommand( new QTreeWidgetItem(), 0 ) );
stack->undo(); // insertTopLevelItem( 0, newitem );

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Result:

NewItem
A
B
C
D

2:

InsertCommand *cmd = new InsertCommand( new QTreeWidgetItem(), 0 );
cmd->undo(); //insertTopLevelItem( 0, newitem );

InsertCommand *cmd = new InsertCommand( new QTreeWidgetItem(), 0 );
cmd->undo(); //insertTopLevelItem( 0, newitem );

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Result:

NewItem
B
C
D

Why I have two different results. What's the difference between 1 and 2?