Almost if not all the magazines and newspapers make most of their income from advertising so if they increase their readership they will get more advertising dollars. So they have a vested interest in increasing the readership figure.
Sometimes these methods maybe a bit too much and although American, I think you will find this article titled “When newspapers fudge the numbers” disturbing.
[Update 11/09/2018 link no longer works]
Probably in response to such stories the ABC here changed its circulation methods.
If so, this has created another major problem that we no longer have any historical comparisons of circulation figures from previous years.
Am I being cynical in wondering if that is partly the reason that they did change the method as surely they could still give us a decent figure or estimate using the old method for comparisons?