Total Tolerance of an Electonic Circuit
These assumptions on standard deviation are only true if the spread of values is purely statistical. In many real-world cases, distribution of actual values is far from gaussian. Several reasons for that:


In many manufacturing processes the difference between two components produced in the same run is rather low, but the run-to-run spread is large. Thus, components from the same reel will have a small spread, but identical components bought a week later might be at the other edge of the allowed values. 
Especially more expensive items might be tested one-by-one and every one that doesn't meet the specification is discarded (or sold in a lower grade bin).
This leads to the fact that in lower grade bins it is unlikely to find a component with a precise value (as this would sell in the higher grade bin) but rather many that are off from the nominal value but still in the specs.