The personality may remain the same but the manner in which one deals with people is different on forums than in person unless you know (socially) the person you're talking to (on a forum)..
Or it may be perceived as such because there's too much room for (mis)interpretation in the written word.
I understand what you mean, but are you talking about your personality or your demeanour?
A couple of forums have a large number of participants who know me pretty well and have done for a long time. Likewise a community forum where I actually know perhaps two thirds of the participants.
There is a difference in the way I behave amongst 'real' mates to complete strangers. Not a conscious change, just a normal adjustment. Less can go unspoken with people who are less familiar with you.
Then there is the context. If I am at a gig my behaviour is pretty distinct from a funeral.
It would be more strange, and less natural, to behave exactly the same in all circumstances despite the company you are in.