Thanks for asking. Too bad my answer is so poor.
The same reason no two cars are exactly the same.
In a revolver, no two chambers in the cylinder are exactly the same.
But the manufacturers try to make them as identical as they can, at reasonable cost.
If you want more detailed and specific information than my first line, you have opened up the wide, wonderful world of interior ballistics and the factors are nearly infinite.
Rifle barrels (even of identical dimensions and weight) vary in stiffness and the resonance of harmonic vibration (barrel whip) will be different. How the barrel is bedded makes a big difference, too. The tools that form the chambers wear as they are used and no two chambers are likely to be identical, though they will be close. Wear on the locking lugs of the bolt changes over time, even on the same gun.
Handguns may seem to be more identical, but only (my opinion) because their shorter sight radius makes it harder to detect the differences, especially when it comes down to analyzing the groups on target.
You have asked a good question. So good, in fact, that it has been a lifetime study for thousands of gunsmiths and ballisticians.