As of now, number of posts in different distributions on linuxquestions.org show:

However the rank and downloads at distrowatch.com shows:
Why this discrepancy? Which one is being used more and why?

Well these forums are linked to us from Slackware.com itself as Patrick Volkerding's recommended forums to go to since the forums that were originally on Slackware.com got a lot of spam. Slackware, having since existed since 1993 had a lot of people move over here with that notice and is the "official" forums for Slackware. Most other distributions have their own forums (Gentoo, Ubuntu, etc.)

As for distrowatch's ranks, those are the downloads from distrowatch.com itself I believe, even though you are able to download the .iso from the official distro's website and some mirrors.

And the number of times a distro is downloaded bears little true relationship to the number in use. That is only the numbers (as indicated) for distrowatch's servers, and doesn't include downloads or purchases from other sources. At one time or other I've downloaded every major distro and quite a few less mainstream to try, test, experimenting, etc. But I actually USE Slackware on a daily basis.