ETTL is a good tool but it as a computer trying to make it's best guess of what you want. In ETTL there is a pre flash before the actual flash. If your lens provides distance information it will do so as a confirmation when the flash is at 45 degrees and pointed forward. As soon as you tilt or swivel the flash distance information is disabled.
When you bounce ETTL is supposed to compensate but I would not trust it. Are you bouncing from a wall that is 5 or 25 feet away? Your cameras light meter has nothing to do with flash exposure but ETTL does use the system to determine exposure. Depending on your camera you can make a few changes. Typically the flash is set to evaluative but you can go into the menus and change to average. In evaluative mode the pre flash compares the ambient reading from the cameras light meter and the pre flash, isolates the subject and then outputs the correct flash power for the subject/s exposure. In average the system averages the whole scene. Ok in tight situations but no so good in large areas like a convention centre. Some people think average is better but I found the difference not substantial. There is one more thing you can try. FEL. This turns your flash metering to spot metering. Some people use FEL and meter off skin tones and adjust the FEC as required. I don't like it because it fires a pre flash and the waits for you to press the shutter. I always had to tell people to wait as the first flash was not the actual exposure.
Back to ETTL and the cameras exposure system. Like I said the system does not use the cameras light meter but does use the pre flash to determine correct exposure. A bride in a white dress, a groom in black tux and the bride and groom together will all reflect light back differently. So like your cameras ambient light meter that will expose to middle grey if you take picture of black tar or white snow the flash system will under expose the bride and over expose the groom.
So there you have it. Using ETTL you will always have to shoot, chimp and fine tune the FEC. Your histogram is your best friend when shooting in ETTL. Now that you understand it you can make better judgements to predict. When bouncing you will have to increase the FEC most of the time. When I have no choice I shoot in ETTL but will always choose manual flash if have the time to set up and the environment will not be changing.
Here is a good link for how to use the histogram. Scroll down and look at the look at the gent holding the white towel.http://super.nova.org/DPR/Histogram/