what did sherlock not know that shocked watson in chapter 2

Holmes listened to Watson and then told him that he was in fact the author of the article and that his job was a "consulting detective." This, in essence, meant that he was a private detective and he endeavored to help government detectives sort out their evidence in crimes. Those clients he mentioned were people who came to him to ask for "enlightening" on some problem they had. For more complex cases, Holmes would visit the site itself and apply his specialized knowledge and powers of observation. To make this clearer, he explained to Watson how, upon their first meeting, he had known Watson was just from Afghanistan.

Watson was surprised at these revelations and commented that Holmes reminded him of Edgar Allen Poe's Dupin and Gaboriau's detective Lecoq.