Real Gun,
The SAAMI standards employ a standard engineering drawing practice for critical dimensions called unilateral dimensioning. Where one dimension limit has to be observed in order for the device to function at all, that limit, and not the usual +/- middle value, is the dimension given. Tolerances are then expressed only in the direction away from the limiting value, depending on which direction you can be off and still have the device work.
In the case of cartridges and chambers, there is a maximum cartridge size that will fit in a minimum chamber, and a minimum chamber size that will accommodate a maximum cartridge. Thus, maximum linear dimensions are critical for the cartridge to work in all chambers intended for it, and the minimum linear dimensions are critical for the chamber to work with all cartridges intended for it. The cartridge linear dimensions are therefore expressed as maximum values with a minus tolerance, while the chamber dimensions are expressed as minimum values with a plus tolerance.
In the case of the .45 Colt cartridge, the cylindrical OD is given as 0.480" with a tolerance of -0.006". That means an actual case may be anywhere from 0.474" to 0.480" OD. What number you actually get then depends on the wall thickness of the brass. It varies by brand and sometimes by lot within a brand. For resizing, however, the sizing die designer has a different problem. He doesn't know whose brass you will use, so he has to design the die to resize all brass enough to grip the bullet even when the wall is the minimum thickness. So, sizing dies tend to bring all brass down to the minimum OD.