Someone else who took that questionnaire said they also got 50-50 on Hufflepuff and Slytherin, which made them doubt its accuracy. I kind of agreed because... Those houses are polar opposites? (Not doubting your results, of course! Just wondering about their processing method. Still, it gave me the exact right House so I'm still gonna believe it more than Pottermore XD)

I've always thought the distinction between Gryffindor and Slytherin was the least clear, despite them being presented as polar opposites, because the classic Gryffindor self-righteousness sounds like something that could easily go in the wrong direction, or be used to justify a hella lot of manipulation and double-crossing in the name of "doing the right thing."