So, thought experiment time:

You have a thousand people, all of which have a coin. You then ask anyone who feels like it to come forward, and flip their coin. I would expect roughly the same number of heads and tails, but would you? Remember, this is one of those scary self-selected samples.

Next, you have a thousand people, all of which have a coin. You tell them all to flip their coin, and use a magic marker to put a mark on the face that lands up, without looking at the result. You then ask whoever feels like it to come forward, and show you their coin. I would still expect roughly the same number of heads and tails, but would you?

Since people who don't know if they can channel don't know how strong they are going to be, the last thought experiment is equivalent to the situation with the novices.