While words matter, the majority of how we communicate is non-verbal which we don't get too much of through online communication. Without the non-verbal forms of communication at our disposal, it's easier for people to misrepresent themselves in all sorts of ways.
The fear comes from experiencing that phenomenon over and over.
I still remember the very first guy I met IRL from an AOL chatroom shortly after my divorce. He spent a good portion of our first and only date accusing me of misrepresenting myself. At first I didn't get it as I had sent him a photo I had taken the day I sent it, and didn't tell him anything online that wasn't true. I did leave off the zoo thing as that was extremely new and at the time I was pretending it never happened, but he didn't know that nor did he have a way of knowing that.
What it turned out to be was he hadn't gleamed from our online conversations and the headshot I sent him that I was about 5" taller than him, plus whatever heels I wore for our date which back then were probably the defacto 3". It was a date, I'm sure I dressed to impress.
And our date was really unpleasant because of it. It ended with him stomping out of the coffee house saying I'm a "lying piece of shit" to use his words. That was shocking to me, because that is not a phrase anyone had ever aimed at me previously.
Afterwards, I realized it had never occurred to me to mention my height unless asked as being tall is my normal. And it wasn't an issue in any way for my ex-husband or the several guys I dated before that. Granted, I met those men in face-to-face situations so they knew how tall I was before they approached me and introduced themselves.
And while that was accidental and not catfishing, it made me very aware of how easily it is for people, myself included, to plug the holes in information we're missing with assumptions and projections, only to be potentially disappointed down the road when we meet in person.