It's simply due to the export resolution being different from the actual resolution. And for some reason, this is not always set automatically to whatever the footage you have is in. Which is quite idiotic. It must be a some kind of a bug, but I don't know what triggers it.
Anyways, right-clicking the sequence and selecting properties and then opening up the export screen and manually entering the values gets REALLY old once you've done it a number of times. Isn't there a quick way to grab this? There should be a button next to the lock icon that says "grab dimensions from sequence settings".
Any comments on this? Is there a hidden way to do it?