Alrighty, here's some of my thoughts.
There are 3 waterfalls in the E3 background, but only 2 in the Grinch Leak.
'Maybe the camera/snapchat/etc blurred it out.'
Unlikely as it's close to the same color as the other waterfalls and those didn't get blurred out completely.
It's also really close to the other waterfalls so it's not like it's on a completely other side of the image where different lighting could effect it. It's right next to the other one.
This is only one of the many discrepancies in the Grinch Leak background. There 7+ more in there ranging from major to minor.
'Maybe the background changed.'
That's possible but unlikely. The background hasn't changed since E3 and there doesn't seem to be any reason to change it at all. All the changes also don't really seem to add anything to the background, so there doesn't seem to be a real purpose behind the changes that would make them necessary. Also most of the changes in the Grinch Leak are were characters were blocking the background. Which makes the changes even more suspicious.
To me this is reason enough to show that it's fake, or at the very least in desperate need of more proof showing it's true.