Common words has a few additional notes on how the frequency table was constructed.
It's worth emphasising that especially with high frequency words, the exact order of the first twenty or so can vary substantially depending on the method used for counting. It is also possible that some words are excluded entirely.
An obvious example is that sono is translated and counted as 'I am' in the frequency table. Sono in Italian also means 'they are'. Unless the initial body of text is analysed by hand both sono with the meaning 'I am', and sono with the meaning 'they are', are considered to have the same meaning.
Needless to say, these frequency lists weren't generated by hand.