We all see the impossible shots on TV and in the movies that the good guys seem to make unfailingly. Snap shots from the ground level at folks on a roof 50 yards away and the like with a snub nosed revolver or such - Hollywood baloney; but as several have already said - if you want to shoot well you have to shoot A LOT.
Shooting is like any other physical endeavor as far as the muscle memory, visual cues, balance and familiarity by repetition. Ask an NFL quarterback or a MLB pitcher how accurate they are if they don't throw a ball for three months.
Yet, MOST folks, not just cops, seem to think that in a life and death situation where the targets are actually shooting back they will be able to perform that which they would have difficulty doing well under range conditions under less time constraints, not to mention less stress. (as an aside I am constantly amazed at the bragging about shots people claim to be able to make under time constraints and distances - even when they are ON the range where it could be verified; when asked to demonstrate such amazing skills they always seem to have a reason why they cannot do it at that particular place and time but still cling to the notion that they have done it and could do it again if only they had their own gun, or their own reloads, or whatever

).
I've always maintained most people CAN'T shoot. At least not as well as the general population tends to believe. IN MY EXPERIENCE the AVERAGE gun owner spends very little time (if any) in actual practice and working on the fundamentals even if the individual practices regular concealed carry for self defense. They just don't. Those here who shoot weekly or even monthly and expend any significant number of rounds are the exception rather than the rule. This extends to police officers as well. I've known many that only shot their sidearms when required to by their departments for qualification purposes, whether quarterly, semi-annually, or annually. And that usually consisted of no more than 50 to 100 rounds at those times.
Thankfully, in the last 25 to 30 years there seem to be many more "survival oriented" officers on the streets that do pay attention to weapon choice and marksmanship, but looking at the big picture I would stand by my above comments.
If you want to do something well, you must do it often. If you want to do something exceptionally well, you must do it constantly. Shooting is no different from anything else in that regard.
That the officer in the original post hit 3 for 8 at 35 yards while under return fire and was using a major caliber handgun is actually pretty good performance. Add the luck and the fact that he went home and the scumbag went to the morgue, hard to argue with the results. As far as his switching to the 9MM, in a case like this officer, I would tend to take the view it's more the singer than the song. He will do as well or better with a 9MM because it appears he's more comfortable with it and I would wager he PRACTICES religiously.