Welcome to ARR!

Tab targeting works as targeting from left to right on whatever is on your screen. it can be a bit confusing, especially on large battles. a lot of times, when I'm using mouse and keyboard, I a lot of times have to do a bit of camera gymnastics to get only the creatures I am trying to attack in my screen, so that tab works as I want it to. there are ways around this, if you are using a controller, via the target cycling features, or via various targeting macros, such as /tenemy (which targets closest enemy)