Why did my markup's background color changed in Mac Office 2011 Word document?

[Ant]

Hello.

This morning I was working a Word document in an updated Office 2011's
Word, in Mac OS X v10.8.5/Mountain Lion, with its markups which I never
used before. So I might be missing something. The right side's
background, showing history like my changes and comments, were in light
blue color. I saved and exited Word. I returned to it this afternoon,
and its right side's background color was in light red.

Why the color change? No one else edited this document beside me. Is
this by design? According to Word's Preferences for tracking, it is
using half red and blue for me. :/

Thank you in advance. :)
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