Alexa just gives approximation results not exactly. At this time Alexa is most convenient and finest tool to measure website current status and rank fluctuations.
Alexa have it's own algorithms (set of rules) they do not exhibit their algorithms.
Another one thing Alexa matter in search ranking slightly because their system monitor each site keywords niche, external backlinks, streaming websites and traffic statistics of different countries (sorting in descending order).
I'm not saying that it's give us exact search ranking but their results are nearer correct. So you may consider Alexa in search ranking. You can get absolute website analysis estimation through this.