People cannot fully represent who they are online, and their personalities are filtered through others' perceptions.
So, you do not get a full picture of who this person is you are talking to.
If you have known for years, have been observant, and had some conversations, you get a better idea, but still not the full picture.
We get to know people by observing them in real time, day after day, and that is not possible online.
Maybe that is why a lot of online romances come to an end when they actually meet or move in together.
It isn't always that the person misrepresented themselves.
Only, they could not represent themselves fully online, and so when the faults and follies of being a human show themselves, it may be too much for the other person or both parties.