In my experience, panel-based traffic measurement services (SimilarWeb, Alexa, Hitwise, Quantcast, ComScore, Nielsen) are useless when it comes to reality and their primary role serves to make misleading news articles or convince advertisers to waste a lot of money on overpriced media buys.
I'm not the only one with this opinion: https://moz.com/blog/testing-accuracy-visitor-data-alexa-com...