The key difference is: they are from different sources (compared to the ones from the authorized dealers). In general, there is specific authorized dealer for most of the brand names who is the one directly import the products from the head office. Since there is an agreement, the authorized dealer is the appointed company & the products will be sold at a certain level of price.
In some shops, they may import the products from overseas shops (e.g. you may bring in some nice items for your own shop when you are traveling in Japan) In fact, the products are also real, but not from the authorized dealer in Hong Kong.