They're not trick questions: knowing the word won't give you an unfair advantage in answering the question, but if you're unsure then your skills of deucation as well as comprehension are being tested.
One thing though: since the questions in each test are supposed to be picked randomly from a huge bank of the things, is there any mechanism to ensure everyone gets a couple of this 'type' of question? They must make it so for each candidate, two 'what is the main arguement...?' questions, three 'which of these is an unstated assumption...?' questions etc are selected, otherwise it would be a bit unfair if one person got 30 very similar questions, or another got loads of 'unusual word' ones.
iva: Here's a thread about what makes a 'good' LNAT score. The universities each have their own standards though.