Interesting question We were just discussing the same thing over here Google had been doing this for years and the reason is doing that is cause of algorithms that will assign new title to the webpage based on it's content. This is done only whenever webpage title is not matching the page content.
@Marcus is absolutely right! Even some webmasters are questioning Google regarding this issue.
May be Google has a plan of setting up the Meta Title automatically as like Meta Description & Meta Keywords.
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