Online, you are what you type. This gives you *explicit* control over every nuance of how you come across. This doesn't mean you'll know what to do with this control and how to come across anyway you like, but you can learn.
IRL, if you focus on what to say, and talk, you will be giving off all sorts of body language and not even know what you've communicated! (And even if you were very still, that would communicate something itself.)
This matters because WRT this issue, you're freer online. There are much lower costs to making choices about how you'd like to come off and changing to do it.
Some may say this allows thieves to be dishonest easier. Yup. Same people like gun control, because thieves can use guns, and crowbar control too. But the truth is tools and freedom help people, and yes helped people, who are better able to live their lives, are better able to do bad, but they are also better able to do good, and in all free societies today, far more people do good than evil.