Frequency of letters change by the length of the texts?
Letter frequency checks how often a letter appears in your text.  Depending on your text, the frequency of letters will change.  If your text is short and has the name "Alex" in it a lot, it will make the frequency of the letter X much higher than average.  Generally when discussing the expected letter frequency, people refer to the average letter frequency that appears in the english language (http://en.wikipedia.org/wiki/Letter_frequency).  Though for specific texts the fequency might not be close to the average, and with shorter texts the frequency might be dramatically different, overall the frequency of a long passage should be close to the average.