Why are the "common" resistors and capacitors rated the way they are?
For the E12 series the step size is the 12th root of 10, or about 1.2 larger than the previous one, so 12 steps take you from 10 to 100. That goes with a 10 % tolerance: you can always find an E12 value within 10 % of the desired value. That's because \$\sqrt[12]{10} \approx \$ 1.2115, and 1.1\$^2\$ = 1.21.



For example: 18 &Omega; + 10 % = 19.8 &Omega;. The next E12 value is 22 &Omega;. Then 22 &Omega; - 10 % = 19.8 &Omega;. (It doesn't always fit that neatly. The blue line shows a small gap between 12 &Omega; and 15 &Omega;, but most often there's an overlap.) Nowadays 10 % isn't used much anymore for resistors, 5 % is much more common, and 1 % is not that much more expensive. 

That means your desired value won't fall into the 1 % tolerance of the E12 series. For example, if you want a 20 &Omega; resistor the closest E12 values are 18 &Omega; and 22 &Omega;. With a 1 % tolerance they don't come closer than 18.18 &Omega; and 21.78 &Omega;, resp. That's why 1 % resistors are offered in a much larger range, typically the E96 range, which includes 20 &Omega;.

Further reading
EIA resistor values series