Thanks for your suggestion. The readability of a text in this tool is measured by the Dale-Chall Score as described in the introduction section. Of course there are other alternative algorithms but anyway this tool focuses more on counting the number of words in the given text.
You need to be a college grad to understand: "The best word count & character count tool online. This word counter & character counter tool is the web version of the popular Firefox add-on and Chrome extension that provide an extensive report about the number of words, characters, sentences, paragraph, unique words, characters without spaces... "
Is that really so hard to understand (imho: no)? Have you considered using another method to find difficult words?