I wonder if it matters whether people are using their trackpad. I was using mine and immediately figured it out; maybe it's because scrolling horizontally is trivial with a trackpad?
But I will say that on my old HP G60 laptop, I wouldn't have had the same ability (it only had the up/down scroll via the touchpad), and I think alot of other laptops are similar. The scroll experience on my MBA is perfectly smooth.
The last time I saw this, I used the numbers. This time, on a different browser, I automatically went to the left and right arrows on my keyboard. I'd wager it isn't so much about the trackpad, but the programs you've been using lately.