Companies can only make predictions based on what the distributors they work with tell them. Even if Wyrd thought that the numbers they were being told were under, they can't risk ordering a lot more simply because of the costs associated with storing excess stock etc. more than a few gaming companies have gone under through ordering way too much stock to meet an anticipated demand that just wasn't there.
My guess would be that they ordered somewhere between the two figures (their estimated figure and the distributors ordered figure) to try and make a balance.
At least the core rules are available as a free PDF!