Vocabulary has for a while been considered in terms of 'receptive' and 'productive' capacity, with the assumption being that ones 'receptive' vocabulary can be larger, since it is easier to hear/read/understand a word than it is to use it correctly in reading/writing (this is not necessarily the popular opinion anymore [http://www.readingconnect.net/web/FILES/english-language-and...] but may provide the context for the claim about Shakespeare). The notion is that you are able to understand more words than you commonly use in your speech/writing, which is on some level intuitive, although of course it is an empirical question.