That's not how it works. Hawkeye makes a prediction of where the ball is travelling. The further the ball travels from the point of impact on the pads, the more uncertainty in the predicted path. Think of a cone of probability extending from the point of impact, which gets wider the further it travels.

There is no such thing as hawkeye being sure or unsure of whether or not the ball is hitting or missing. There is always a degree of uncertainty. The way DRS has been implemented, Hawkeye simply displays the ball's most likely path, and if this path has the ball hitting the stumps with less than 25% of its diameter then the on field call is deferred to. This is not a mathematically ideal way to implement haweye - ideally, we would also take into account the distance the ball has to travel before hitting the stumps, because a greater distance confers greater uncertainty.

Your interpretation can pretty easily be shown to be false. Say a player is struck on the pads right in front of the stumps. Millimetres in front of the stumps. Hawkeye's ball tracking prediction will therefore be highly accurate, with little uncertainty. However, let's say that the ball is only going on to hit the stumps with 20% of its diameter. In this case, the decision will still be deferred to the on field umpire's call. Hawkeye in this case is not 'unsure' if the ball is hitting or not. In fact it is very very sure of the path of the ball from impact on the pads to the stumps.