Counting visitors is one of the trickiest things you'll see in domaining imo.
Just think about it,
how can you distinguish a visitor from another? what if someone uses a proxy or if there are multiple different people using the same IP?
in short,
you will never have 100% accuracy in counting visitors so everyone have their own way of doing that just to give you a general idea.
What really matters is how many people actually made a legit query to buy your domain (assuming you are selling) or how much conversion happens that sends money in your pocket (if you are parking, affiliating, etc.)
as a final fyi note,
Alexa (and all similar services) have deals with various companies, browsers, registries, etc. To analyze 'hits' but that's only really a guess since their algos use what their partners send them, ie. a really small sample if you think how vast the internet is.
Sorry for the long text but this is a topic that it's really hard to explain in a few words
