[Kaderin Triste]

Well, they do give the full list of questions for the wand one. But for the others, I agree. Because, as Wizardmore's extended sorting hat shows, your likelyhood of being in one house or another can actually be determined by a very slim margin so of they only give you a handful of answers, it could be ones that are more slytherin when the rest of the questions they don't ask you are more likely to put you in hufflepuff.
For being an official site, Pottermore should at least give the option to do a full quiz instead of the short one for more accurate results.

In 3 different sorting hat attempts on pottermore, I've gotten Ravenclaw, gryffindor, and slytherin.