While I don't think the original poster of this thread is necessarily implying that s/he is intending on committing an act of academic dishonesty, the way the person worded it does seem a bit suspect.
They've got a heck of a point. Turnitin.com isn't a very reliable tool. I remember using it several years back in high school (when taradactals were ridden by Fred Flintstone) and it was abysmal. I was accused of copying about 60% of my paper. The site had different colors - I think yellow, orange, and red to symbolize how strongly your sentences/paragraphs resembled something else out there on the web and about half of this particular paper was in red and orange.
I know it did not improve much because by my 3rd semester of college a few years back, the same thing happened in a writing class to a lesser extent. Fortunately, that time around, the professor took a more reasonable approach to interpreting the turnitin.com results...