I was sitting on the bus going to work today debating yet again whether Snape was a goody or a baddie. Just in case you were wondering, I do this at least once a week just to take my mind off all the morons putting their feet on the seats and the kids playing with the emergency exits

Anyway I digress. One huge thought suddenly struck me. Most of the case for Snape being a hero was that he could have fooled Voldemort into believing he was a Death Eater whilst all the time being a double agent for DD. We are lead to believe he could do this because he was accomplished at legilimens and occlumens.
If this was the case, why, when teaching Harry occlumens, did he feel the need to use a pensieve? Surely it would have become clear after the first lesson how useless Harry was - I know Harry glimpsed a young Snape once - but if Snape put his mind to it he should have quite easily been able to block Harry.
Two possibilities have occurred to me
1. Snape is rubbish at mind tricks but pretended to be good so he could act as a double agent for Voldemort. DD would have been far too polite to try and read Snapes mind to see if he was telling the truth.
2. Snape rigged the whole 'Harry sneaking a peak at the pensieve' thing. Maybe he deliberately picked that memory to put in the pensieve and arranged to be called away knowing what a nosey wotsit Harry was. This would serve two purposes; discrediting James (and this sort of worked) and giving him an excuse to stop the lessons.
Whatever the answer I still think it is dead suspicious that Snape was confident about fooling Voldemort but not Harry
