When you''re a skilled and aggressive CB mover, like the pros, you tend to hit a bit firmer and rely on the right draw/stun/follow + spin to float or kill the CB where you want it on many or most shots. When they're tearing through a rack they're really more focused on where the CB is going and how to get it there, with their foot hard on the gas.
What happens when otherwise great shooters miss the last ball is usually that they let up off the gas and/or 'steer' the cue. If they've been hitting repeated firm shots for position, they've been sticking harder into the CB and focused more on their cue delivery to control the CB. When they're just rolling the last ball in and let off the gas, they may subconsciously steer the cue towards the contact point on the OB, which results in a thicker hit.
That's why with really serious players, even if they're close to the pocket with an easy shot, they'll stun or draw even though to the amateur it looks like there's no reason to do so and no risk of a scratch. They're not doing it to make a statement or show their CB control, they're doing it because 99.9% of their mental game is about CB control and if on the last ball they decide to soft-touch it without precisely controlling the CB, they risk having a brain fart where their control breaks down and they steer their cue ruining the shot.
Yes this can be caused by nerves, being timid about the shot and reverting to n00b mode, easing up off the stroke. But with players of this level, it's definitely an execution and not an aiming problem. These guys don't even remember the last time they had to think about aiming, it's like breathing to them, their focus on their shot is always how the CB is going to come off of the OB, not where the OB is going, because that's always a given for them.
Edit: In short, the problem is they've reversed their usual process on the last ball, thinking too much about aiming the OB into the pocket when their mental game is developed to aim the CB to come off the OB and land on some specific spot on the table. They know where to contact the OB without even thinking so when they DO make that what they're thinking about, they suffer a breakdown of the other things they do well, i.e., dead-stroking the CB with the line, speed, and spin it should have.