E-TTL can be tricked into thinking that it's exposing correctly.
Lets think of it like you're just shooting without flash.
When you point your camera out with half of the frame being a bright afternoon sky, your camera will automatically adjust its settings to get you what it thinks is the correct shot (assuming you are in Av/Tv/P or auto mode with evaluative metering). Your picture will turn out dark to your eyes because the camera thinks it has to balance out the white sky to become 18% grey.
Vice-versa with a man dressed in black against a dark background - your camera will expose higher than your eye sees and the black will turn into 18% grey.
For both of the above situations, you'd need to key in exposure correction into your camera. It could be just a third of a stop, or it could be three stops, depending on your situation / frame.
Flash TTL behaves the same way, except that the pre-flash + computer determines the final flash output.