Difference between Comscore and GA data:

Now a days most of the agencies refer Comscore data to monitor the actual traffic, users base on a site which is when compared to site's GA data it does have difference, sometimes the difference can be above 50% too.

When a publisher pitches with their GA numbers with concerns then we generally suggests refer to Comscore data, there are four reason behind this:

Comscore eliminates or ignore the machine generated traffic whereas GA counts all the visits irrespective of bots or real user which will automatically lead to a huge difference.

Comscore only measures people instead of cookies where GA measures the cookie user too.

Also, Comscore eliminates visits that is when people views from multiple browsers and multiple screens (Mobile, desktop, tab etc)

GA uses the real data from your real visitors, it records and collects the anonymous data and provides easy-to-read formats of that traffic information whereas comscore assumes the number based on a pool of poll users that are tracked via plugins,tracking cookies but the guesses are based on the algorithm which is derived after a long research and the final numbers are maximum time same as the actual of the site data