- Computers which are shared by multiple people may have multiple sets of cookies.
- Computers which are part of a local network may have a single set of cookies, per person, shared across multiple computers.
- Some blog owners may use multiple Blogger accounts.
Some computers are owned by, and used by, multiple people. The operating system will encourage each different person to maintain her / his own settings and styles, and to identify herself / himself when starting the computer. The settings and styles are maintained in a personal profile - and most browsers maintain the cookies as part of the personal profile. If two people, who share a computer, also share a blog, each person will have to select "Don't track ..." consistently - or face having inconsistent counting of pageviews, when reading the blog.
Some local networks, where various computers are shared and used locally, may use profiles which are maintained in common between the various computers. Changes to the profile (including cookies), made on one computer, may transfer to other computers. Clearing or setting cookies on one computer may affect presence of the same cookies, on another computer - and may again cause inconsistent counting of pageviews, against blogs involved.
Some blog owners may use multiple Blogger accounts. Similar to the issue of blogs shared by different people / used on shared computers, blogs read on computers used by people with multiple Blogger accounts will have the "Don't track ..." cookie present, irregularly. This, too, will cause inconsistent counting of pageviews.
Finally, as noted, clearing of cookies will affect presence of the "Don't track ..." cookie - and will cause unexpected counting of pageviews. This inconsistency will be more common with computers shared by multiple owners, and with computers shared across a local network.
Many blog owners use only one browser, and one computer - and own and use their own computer, exclusively. Any blog owner, noting inconsistent effectiveness of the "Don't track ..." option, however, may do well to at least consider the above issues, occasionally.