My pesecetarian friend calls herself veggie to other people because a lot of people don't know the word 'pescetarian' and get more confused when you use it than the term 'vegetarian who eats fish'.
Still. She's a pescetarian because she thinks that, like, cows and pigs are born and raised to die, but fish aren't, usually (fish farms are an exceptions). That's why she eats the fish but not the other meat.