Like about every other part of a gun, chambers are made to a +/- tolerance. The reason is that they use reamers and need them to last a while. So they start with the reamer at the outside edge of the tolerance. After it wears, they sharpen it, reducing the hole diameter slightly. When the reamer is worn to the point that the hole is at the minimum edge of the tolerance, they scrap the reamer and start with a new one.
The same is true of the reamers made to cut rifle chambers, ammunition manufacturing dies, reloading dies, etc.
Actually, people get a bit over-wrought about revolver throats. The fact is that a thousandth or two in a .45 won't make any difference. A lot of folks who worry about throat diameter shoot cast lead bullets that can vary more than the amount the throat is likely to vary.
(P.S. I would like to blame the throat diameter when I can't hit anything; problem is that it's me!)