I don't think that they seemed that bad at either, they were just in very tough conditions. In the maths test, they had paintballs thrown at them, and in the spelling test they had to find the letters in something disgusting, whilst against the clock in both. I think if they were relaxed, the Housemates would have done a lot better in both. I think most people would make elementary mistakes in that situation.
Having said that, Danny did seem very poor at spelling, but maybe he's dyslexic.