I'm no physics genius, but the sensaiton you feel probably has something to do with the fact, while gravity is technically the same, the length of the log (and the distribution of the weight contained therein) might make the log start to drop while you're already sorta kinda hanging out in the air.
Balance a penny on the edge of a butter knife, and sloooowly start pushing the butter knife to the edge of a table, blade first. Once enough of the knife is off of the table, the knife will fall in an arc, because there's still some of the knife on the table, whereas the penny will drop more or less straight down.
Now put the penny on the knife's blade again, but slooooowly push it off the table, handle-edge first. Because the penny is on the last part of the knife to be affected by the pull of gravity, you'll see a little more of a catapult effect.
Similar principles are at work on Splash Mtn, except the drop isn't as sharp, and because you're inside the log, with at least part of your body more or less pressed against it, you fall with it more than the penny on the knife's blade. But it could still gives you an almost-zero-gravity sensation as a result. It'll depend, mostly, on how disproportionately the weight is distributed inside the log, and where you are seated. You may feel more of a direct drop. You may feel a little more of a catapult. The differences will be slight, but you're already jazzed up in anticipation, your senses are heightened, it seems much more different.