You don't say如何理解？

情景会话1 表示惊讶

Rebecca: I ran into my ex-boyfriend at the bar in Tokyo last night. （我昨天在东京的酒吧里撞见了我的前男友。）

Zoey: You don't say! （真的假的啊!）

情景会话2 表示讽刺

Zoey: Peter failed the exam again. （Peter考试又挂了。）

Tim: You don't say. What's that, four in a row? （可不是么。这第几次了，连续第四次？）