What the report is based on: a median value selected from the traffic to 42 of the top US websites (it ignores those which don't have both and app and a website) from data provided by Comscore.
The term "traffic" is never defined. Web requests? Quantity of data transferred? Both of those would have significantly shapes depending on whether the client was a browser or an app.
It's also interesting to note that the top 10 "Browser Reach Advantage" websites are basically text based websites - news sites, wikipedia and blogs.
Things I learned from the report: the top website (with 207.0x Browser Reach Advantage) Blogger has an app.