What is ‘Last Modified’ on post list page meaning?

I’ve just noticed that on the ‘Date’ column of the post list page, both draft and private entries have ‘Last Modified’ notes, however the time/date shown above the notes aren’t the exact time of my last modification, more likely they are dates of when the posts are created.

For example, if now I have modified a post which was created 2 days ago, on the post list page at the row of this post it still says:

2009/02/16
Last Modified

So I’m confused, what is this ‘Last Modified’ meaning?

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