Um. Why carry something you don't shoot best?
There are a bunch of reasons. For instance, if you shoot a full size gun best, as many of us do, but are going some place that is definitely gun-unfriendly, you might carry something that conceals better. Or, the gun you shoot best might be a bear to carry all day long. In my case, the gun I shoot best has crappy sights that work well on the range, but would be near useless in a low light situation.
I'm not talking about carrying a gun you don't shoot well, but just wondering what the rational was when someone carries something different than their best. In my case, I think it comes down to caliber. I shoot 9mm best and I really feel that 9mm is more than adequate for daily carry but sometimes I want something a bit bigger. I've found myself doing that since last summer after the theater shooting. I know it's silly, yet I still sometimes go with a 10 or 15 round capacity 10mm just because I'd want as big a hammer as possible in that situation.