Those questions are not representative of the USMLE Step 1 Questions. The reason they have it out is for you to see how they will pose a question, how they may present pictures, etc. The questions that ARE representative are the questions that you buy from the NBME (forms 1-4), those are quite representative, and some may even show up on your real exam (but at $45 a pop, is it worth expending ~$200 to get maybe 10 questions verbatim on your exam?). I got this info from my upper-classmen friends who've already taken the exam, I'm pretty sure it's true.
Another reason they made the questions available is for you to try out their FRED software (now with video, but that won't appear until after my Step 1 exam, so for now, WHEW!). While it does give you a score and all, really it's more of a "you're on the right track keep studying" deal, don't think of it as an indicator of your real exam! QBank may be harder, but it makes you search out those little details that will help you on your exam. I hear that UW is good and their questions are on par with the exam, which is a plus and minus here. Take all this with a grain of salt.
Also, Kaplan posted (I think for free) the explanations for this practice 150q test.