I believe the general consensus is that the Whizlabs exams are slightly harder than the actual SCJP. And, yes, a small part of that added "difficulty" does unfortunately stem from the occasional ambiguous/poorly-written question. But on the whole, I think their mock exam questions are actually very good in terms of how well they represent the actual exam.
And, speaking from my own
test experience, even the actual SCJP questions aren't always that clear or concise. So just grit your teeth and try your best to discern the intentions behind the ambiguously-worded questions. If nothing else, this'll make you a battle-scarred veteran by the time you step onto the real battlefield.