I think the anonymity involved in many online interactions -- certainly including forums like this one -- emboldens liars in their lying and truthful people in their telling the truth. Not that anyone is entirely one or the other, of course. As I believe Dostoevsky pointed out in
The Brothers Karamazov, the line between good and evil runs through every human heart.
I dunno. I guess I've never really thought about it, but to put in Christian terms that the OP may or many not be able to relate to, if I had to guess I would say that I know I have enough "real world" sins to deal with without adding "lying on Christian Forums" to the pile. It just seems so unnecessary. Lying for what? It's not like if you get people to believe lies when they have no reason to not believe them (say, about your own background, which logically only you would really know the truth of), you somehow gain something out of that. Sure, you got them to believe a lie, but it's not exactly an act of cunning if it occurs in a context where people are likely to believe you anyway.
This sort of thing is part of the reason why I've gone into a sometimes uncomfortable detail about my own background, even though I personally do not like doing so. There is the so-called "Principle of Embarrassment", whereby people engaged in historical criticism suggest that the more embarrassing (a.k.a., humanizing) details in a historical figure's record are more likely to be true than the hagiographical details. The thinking goes that the people attempting to mythologize or sacralize a figure would be unlikely to include unsavory details about them unless they were relatively certain that said details reflect the truth in some way. How would the early Muslim community, just for example, benefit by passing down information about Muhammad marrying so many women? Of course, we know that in that specific context, it is seen as fine that he did that, but we also know from early Christian writers (both those who came into contact with the early Muslims and those who did not) that this was a point on which Islam was criticized for its somewhat regressive moral values, relative to both the sayings of Christ as preserved in our religion (recall His treatment of the woman at the well who had both many and no husbands), and to what was established as normative behavior centuries before Islam was ever around. St. Basil (4th century), for instance, calls polygamy "fornication".
So I figure, by the same token, whether or not anyone thinks I'm lying about anything, if I include things that would make more sense to not include, then at least people know by inference that I'm
less likely to be lying.
Plus, who pretends to be Coptic Orthodox, of all things? Eating beans and pita bread for most of the year isn't that glamorous, folks.