IPSC Production (and I or CDPA and every other gun sport) is target shooting. You can call it as close to duty or defensive or reality as you want, but in essence, you're target shooting. Hence the difference.
Duty guns put in close-up fixed sights, which are fine for the gun under stress at 5-7 feet, but useless in hitting a pepper popper at 15m on the run compared with a set of target sights. Target sights, on the other hand - are more susceptible to damage due to lots of parts.
Duty guns have heavy triggers to compensate for life-in jeopardy-adrenaline pumped motor skills that materialize when you least want them. Target pistols have fine triggers. Paper doesn't shoot back. Gun sports are never the same no matter what fantasy some might spin in the stage design and briefing.
Duty guns are designed with a lifetime round count of 20-30K rounds. Ask an average cop how many round he puts through his pistol and he'll ask you what the qualification round count was - and yes, I know some guns go longer but that is thier specs to meet. Competition guns are designed with a far greater life in mind. STI expects you'll shoot an Edge 20K round a year or more.
Competition guns get cleaned before and after competition. They travel in nice cases and live in nice safes, duty guns go in holsters, and sometimes stay in there for months, being dragged into every environment known to man, again only getting cleaned at qualification time. A Competitive shooter treats his gun carefully - he wants to shoot. An average cop considers it more weight on his belt, and generally a PITA - he'd rather not have to shoot it.
The closer you move "duty" people to intensive shooting, the more they ditch the traditional "duty" gun and go to something more competition oriented. There's a few special forces organizations that use the STI Tactical, a slight derivative of the EDGE. At thier level of proficiency, they properly maintain these tools and in a firefight, they don't tense up like the average cop. They're also less likely to get sued after a shooting. There is even a a SWAT team in Asia using IPSC race guns as entry weapons.
And finally, there's always the budget - competition shooters have a far bigger budget than a police force for firearms, so target shooters can simply afford better.