[chongli]

If you have 2 identical resistors that are 5% over nominal and you put them in parallel, you'll get a value 5% over nominal. Example:

Suppose you had a pair of 105 Ohm resistors that are nominally 100 Ohm. In parallel you get:

1/(1/105 + 1/105) = 105/2 = 52.5 Ohm (5% over expected 50 Ohm)

If one is over nominal and the other is under, they'll cancel out for the most part:

1/(1/105 + 1/95) = 49.875 Ohm (0.25% under expected 50 Ohm)