In most cases Arduino hobbyists rely on power supply reference when measuring analog values. If we take 5V Arduino connected to USB intuitively we assume that its 5.00V. But in reality voltage value may be different that lead to quite large errors in measurements. Matt gives a tip on how to avoid this. First of all he suggests finding what exact VCC value is using simple technique found on TinkerIt site.
Microcontrollers Atmega168/328 in arduino have internal known (1.1V) reference voltage which can be measured with ADC against unknown VCC. Then with some calculus VCC can be corrected to exact value. Having precise VCC value measurements become closer to expected.