Yes, you are missing something.
Just about all of the "major" differences between trumpets are in the 1/100 to 1/1000 region. Bore size .458 is considered "medium large", .462 is large. A 7C Bach mouthpiece has an internal rim diameter of .652, a 1.5C is .660.
If you take a look at the link that I mentioned, you see how even a bit of sloppy silver plate can make a difference.
Everything makes a difference, very few know the real reasons and most just tend to believe myths. The biggest and most common issue in the horn is it not being clean. That can often be in tenths of an inch!