It actually doesn't. Believe it or not this guy is not the first person to realize that not all 26" wheels are going to be the same size thanks to different volume tires.
26 or whatever wheel size you like is only a vague guide, the actual size for a wheel is designated by the rim, i.e. 559 for a 26, 700 for a 29 etc, these are international standards that are measured in metric, so the conversion to inches, which then includes tyre size is only approximate, hence the variety in actual size.