1. Number of visitors as recorded in Google Analytics
2. Number of loads of a 1x1 pixel served on a different domain
They see higher numbers for (2) than (1), and attribute the difference to users blocking Google Analytics.
I don't see them describing how they excluded bot traffic, however, and for my sites the majority of hits I get are from bots. Only some bots run JS, so I suspect their numbers for blocking users are thoroughly diluted by these bots.
(Disclosure: I work for Google, speaking only for myself)
1. Number of visitors as recorded in Google Analytics
2. Number of loads of a 1x1 pixel served on a different domain
They see higher numbers for (2) than (1), and attribute the difference to users blocking Google Analytics.
I don't see them describing how they excluded bot traffic, however, and for my sites the majority of hits I get are from bots. Only some bots run JS, so I suspect their numbers for blocking users are thoroughly diluted by these bots.
(Disclosure: I work for Google, speaking only for myself)