收益率曲线yield curve上升，roll down return的变化？

这说的是随着时间流逝，收益率曲线向上倾斜，折现率下降，rolldown return上升。

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而题库有一道题是说：

how the expected total return results would change if THB yields were to rise significantly over the investment horizon?

答案是

The Buy-and-Hold expected portfolio returns would be unchanged and the Yield Curve Rolldown expected portfolio returns would decrease due to the rise in yields.

解析：

rolldown return是沿着收益率曲线，用两个不同的收益率计算的价格，带来的return

如果收益率曲线向上，那么期末折现用的折现率更大了，算出的价格更小了，所以rolldown return更小了。

这两个地方不是矛盾了吗？请老师解惑，谢谢！