Problems building expression tree from postfix notation

I'm currently writing an impreter for simple mathematical expressions (constants and simple arithmetic).

The problem I'm having is with building an expression tree from a postfix formatted expression. What I've done works fine in most scenarios, but not with this example from Wikipedia.

If I evaluate the expression 3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3, I get the result 3,0001220703125 even though the result should be 3,001953125. The reason for this seems to be that the expression tree looks like 3+((4*2)/((1-5)^(2^3))) instead of (3+((4*2)/(((1-5)^2)^3))).

The postfix notation of the original expression looks like 3 4 2 * 1 5 − 2 3 ^ ^ / +

Any suggestions how to get the expression tree as I want it to be?
Below is the postfix to expression tree code and some tests which is in C# but should be pretty self-explanatory.

public MathExpression Parse()
{
    var tokens = this.ToPostFix(_tokens);
    var stack = new Stack<MathExpression>();
    foreach(token in tokens)
    {
        if(token.IsOperand())
        {
            // Push the operand on the stack.
            stack.Push(new ConstantExpression(token.Value));
        }
        else
        {
            Debug.Assert(token.Type == TokenType.Operator, "Expected operator.");
            var op = (Operator)token.Value;
            var right = stack.Pop();
            var left = stack.Pop();
            var expression = new ArithmeticExpression(op, left, right);
            stack.Push(expression);
        }
    }
    Debug.Assert(stack.Count == 1, "More than one expression on stack.");
    return stack.Pop();
}

And some tests:

[Test]
public void Wikipedia_Example_Can_Be_Evaluated()
{
    var expected = 3+4*2/(1-5)^2^3; // 3,001953125
    var actual = MathExpression.Parse("3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3")
                               .Evaluate(); // 3,0001220703125
    Assert.AreEqual(expected, actual); // Not equal :(
}

[Test]
public void Can_Convert_To_Prefix()
{
    string expected = "3 4 2 * 1 5 − 2 3 ^ ^ / +"
    string actual = MathExpression.ToPostFix("3+4*2/(1-5)^2^3")
    Assert.AreEqual(expected, actual); // Works as expected
}