Distrowatch rankings are not that useful. Sometimes incorrect statistics are worse than no statistics at all.
The problem with DW page hit stats is just that - they are based on page hits alone. They also (roughly) measure popularity, not necessarily quality. So it's not a "best distro" barometer.
The first problem:
Some distros are the first port of call for casual, experimental or one time users. Ubuntu and Mint fit this category well. If a new member, new to GNU/Linux arrives on this forum looking for a starting point people recommend ubuntu or mint - that's more page hits for ubuntu or mint.
The second problem:
There are rolling release distros where the system is in a constant state of flux - there is no release as such, the user simply upgrades via the package manager and does not have to go to the distro's website and download a new .iso - that's less page hits for those distros.
The third problem:
There are distros which only release e.g every two years and which have a solid upgrade path - meaning less people downloading .iso images from the distro's web site, meaning less page hits.
The fourth problem:
There are rapid release distros, e.g. every 6 months, where users will tend to visit the site and download iso images more frequently. More page hits for those distros.
The fifth problem:
There are users who make use of mirrors or torrents and may not necessary visit the distro's site to download the new release. Less page hits.
The sixth problem:
The "veteran" users tend to reinstall much less often, instead preferring upgrading to the next release, they're also less prone to "distro hopping".
The seventh problem:
Noobs tend to break things and have to reinstall more often - in particular they tend to break upgrades or just not attempt upgrades at all.
The eighth problem:
Some distros are available as livecds, some are not or only via 3rd parties. Livecds are useful and used by many from demos and for recovery,. The downside is that livecds cannot be just upgraded - you have to download a new .iso when there is a new release. This means that distros which provide livecds will see more hits.
The ninth problem:
Some distros provide special desktop specific spins, this can mean (in particular in the case of Ubuntu) that the different spins are listed separately on DW - this means that hits are split across a few separate distros which are really one distro, meaning lower ranking.
The tenth problem:
The 'fanboi factor'. To avoid flamewars, I won't elaborate too much on this, but let's just say that users of certain distros may have worked over time to push those distros up the rankings at some point.
I also don't think the number of people on a forum is in any way useful in determining popularity. Certain distros which appeal to or are designed for newer users, will of course see a lot more forum traffic.