That's because creating an account on the website is mainly aimed at current and potential owners of Arturia products, whereas forum members aren't required to be owners of Arturia products. The two things are completely separate, hence the separate logins.
In fact it's actually quite generous of Arturia to allow non-owners so much potential free-reign on these official forums. Some other manufacturers' forums - Presonus for one - don't allow you to even post unless you are a registered owner, and even then they only allow you to post in the specific subforum(s) related to your product(s). The trade-off for Arturia's generosity is an increase in spam-posts (which Arturia staff and volunteer-mods like myself do our very best to keep on top of) and the fact you have to sign into the website and the forum separately. Not exactly a drama is it?
