If in the 1800’s ordinary people had a vocabulary of only 3000 words, and the illiterate had only 500 words from which to choose for use in their limited vocabulary, it’s no wonder that they had difficulty understanding what the well-educated had to say with a vocabulary of 5000+ words. How many words do you use on a daily or even annual basis? That is to say, how many unique words, not how many times do you use the words a, an, the, and other such words.
From “How do I Pronounce?” 1885 by Phyfe: 118,000 words were in the dictionary in the 1880’s. 30,000 words were in practical use. 15,000 words appear in Shakespeare’s works. Cultivated persons in general use only 5,000 words. Ordinary people use 3,000 and the lower end of the illiterate population have the use of only 500 words.
According to Adolphs & Schmitt (2003), 2000 words can provide an understanding of 95% of conversations. The typical conversation requires only 90% to 95% of the words to be understood in order to understand the conversation as a whole. (van Zeeland and Schmitt 2011)
How many more words have been added to dictionaries since the 1880’s? The Second Edition of the Oxford English Dictionary contains 228,132 words defined either as main entries or as subentries. Close to 50,000 of those words are considered obsolete. (from oxforddictionaries.com) It seems that even though the number of words in the English language has increased, the number of words used on a daily basis has not.
Judging by this information, it is easy to see how the common person (high school educated) could have difficulty understanding what is being said in contracts, legal briefs, and other such works.
Note: I’m terrible at citing sources, but the curious have their ways of finding the information alluded to. I’m just trying to point you in the right general direction.