Actually the OG-FG method using a hydrometer is NOT completely accurate. Just so close to right that nobody needs to worry. The reason it's not completely accurate is because as has already been mentioned you are measuring the total density of the wort/beer not just that of the water and sugar(OG) and then the same water minus the sugar(FG). Other compounds which can affect the density of your sample also drop out of suspension over time as well as the fact that alcohol has a different specific gravity compared to water which throws your calculation off because the FG isn't just water minus the sugar of the OG, it's water and alcohol minus sugar. But everybody is right in that it's the way to go. It's cheap and as accurate as homebrewers need to be, even for competitions. Ie if measure and calculate a brew to 5% and it really turns out to be 5.09, do you think anyone can tell or will even care?