According to Shog9, "last seen" is only updated every 15 minutes and thus is not as accurate as other timestamps (e.g. post and comment timestamps):
The "last seen" date is only updated every n minutes, where n was 15 the last time I bothered to check.
So when this time is recorded, it won't be updated again for at least another 15 ...
The last seen is inaccurate intentionally, as mentioned in Laurel's answer.
The timestamps on comments, questions, answers are precise. For recently posted ones, the time will be shown as x mins/secs/hours ago, as seen in your screenshot, and when you hold your mouse pointer over it, you'll see the UTC timestamp: 2017-10-20 21:44:42Z.