Your "wasabi" is lying to you. That green stuff you get in all those questionable prepackaged boxes of sushi from your local grocer? It shouldn't come as much of a surprise that that ain't wasabi, but it turns out that most Japanese restaurants aren't giving you the real stuff either.
According to The Washington Post, most sushi wasabi is actually a horseradish-based paste that's colored with yellow and blue food dyes and injected with acids and mustard extract, and then unceremoniously plopped on your plate. That's because real wasabi is apparently extremely difficult to cultivate, and therefore very costly -- and Japanese chefs the world over came to an implicit agreement to just use this other stuff.
Real, green wasabi mostly comes in the form of a stalk that's freshly grated, has a sweeter flavor, and is rare even in Japan; experts estimate that roughly 95% of wasabi in Japan is fake, compared to a staggering 99% of American wasabi. Of course, you can always find the authentic stuff if you want to -- you'll just have to shell out a little more green.