Reasons for the dirrerence in rated and measured capacitance?
Answer:
Manufacturing processes are not that precise. When capacitors are manufactured, variations in thickness of the bulk materials, different pressures fitting components together, variations in temperature on the factory floor, and other factors, all affect the value of the capacitors coming off the 'line'.
For a particular value of capacitor, the design of the manufacturing line will produce a capacitor of that value -- NOMINALLY. In other words, statistically the value of the capacitance will be within the stated tolerance of the specified nominal value.
When you get a single capacitor and it is different than the value marked on the package, it should be within the tolerance stated (in the manufacturers datasheet or printed on the body of the capacitor). If the value is not exactly the value printed on the capacitor, then it's because the foil was a little thin on that portion of the huge bulk roll of foil that day when the cap. was produced. OR, it was a little hot in the factory that day and most of the capacitors came out a little on the low side, OR... you get the idea.
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How much difference? I have seen 1% capacitors as small as 0.8 pF. When measured on an expensive Agilent bridge, they would measure 0.79 or something. Incredible.
Many electrolytic capacitors used for filtering have terrible tolerance, as much as -20 +80%. For filtering its OK. You just have to know the spec of the device.
0.1uF caps in ceramic are also low in tolerance, maybe 20%, because they try to pack so much uF in a small space.
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