Any idea why the estimates GWT has given me are so wrong?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  Tim Quin: Happy New Year everyone!
  
  Just a quick one. I just noticed when looking at GWT that my client's site is now ranking 1.0 and 1.8 for a couple of search queries that relate to a blog post I wrote. But when I actually search in google for these terms I'm nowhere to be seen in the first five pages. Any idea why the estimates GWT has given me are so wrong?
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  George Buchan: If someone comes up with an answer, I need to know this aswell lol, ;
  If what you see in WMT were true i would need to be an Octopus to cope with all the work!!,
  
  Sadly WMT is full of Sh*T ....
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  Greg Kristan: I am going to take a guess since I have seen this before.
  
  Perhaps the blog was some sort of localized or personalized search for that user if they were signed into some Google property. Therefore when they searched in Google, Google put the result at 1.0 and 1.8 on average. However for the rest of the users this is not a personal search or local search and that is why you can't find it.
  
  Totally a guess but If anyone has the real answer please tell us!
  
  Great question!
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  Brent Wildman: +Greg Kristan ;is correct. Local and personalized results will skew these numbers. It's impossible to tell where the searcher was searching from, what device they were using and if they were logged in or not. ;
  
  Also Webmaster Tools deals in averages and approximations. So if your result was between 1-2 for the last 20 something days but recently it completely dropped off, it either wont effect the current average or effect it a minimal amount i.e. 1.0 to 1.8
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  Joseph Paulino: +Tim Quin I agree with +Greg Kristan and +Brent Wildman. Also WMT doesn't tell you when exactly your blog post page went visible in the SERPs in that range of ranking average. I am guessing it would be that it dropped of in rankings after a short time of being there on top even with personalized, interest based, and/or localized search result because a majority of users went back to Google and modified their queries.
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  Tim Capper: It is a combination from different locations across the web. Use the filter to turn on your exact country search. You should see a difference.  ;
  
  Also WMT is like 30 days out of date. So if your blog post was there and not now, it shows you how out of date it is.
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  Tim Quin: Thanks guys, this is helpful. So let me just check I understand this. I wrote the post that these searches delivered 8 weeks ago. So bearing in mind what Tim says about WMT being about 30 days out of date, at some point in the first four weeks of this post's life a fair amount of people (the impressions for each were 50 and 70 respectively) have searched for the two phrases in question. And based on their location, personalized search, what server delivered their results etc. my post got these average positions right?
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  Tim Capper: Thats is about it in a nutshell ...... I think :)
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  Tim Quin: Great, so my takeaway from this is; WMT is pretty shit at telling you how you are ranking!! Advice on getting a more accurate idea anyone?
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  Brent Wildman: There's tons of different rank checkers out there. I personally use Advanced Web Ranking, it's a little pricy but it's very accurate and exports tons of different reports.
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  Tim Capper: Webmaster tools just had an update and the data looks pretty accurate on rankings
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  W.E. Jonk: From the expert panel in this weeks SEO Questions hangout on air on 01:35:51 into the YouTube video: https://dumbseoquestions.com/q/any_idea_why_the_estimates_gwt_has_given_me_are_so_wrong