Why does this happen?   If a light ray hits a window at 90º, it does not get bent, or refracted, at all.  If it hits at an angle, however, it IS refracted.  One way to understand this is by studying an analogy.

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Here is my rendition of the "axle analogy".

The red light ray comes from air into glass (entering more dense stuff) and gets refracted BOTH when it enters AND when it exits the glass

The wheels are there to show you why this happens... the bottom wheel hits the glass first, so it gets slowed down first (first purple arrow)

The ray goes straight through the glass

The top wheel gets oout of the glass first, so it gets t speed up first (second purple arrow)