When someone comes to your website and wants to save one of your pin images (you have those on your website, right?!), the default image is saved with titles and copy that is dictated by your blog. An excerpt, for example, would probably be what shows up in your pin. You might not realize that when someone saves your pin, the default is used. Unless the pinner edits that pin (description, title, hashtags, URL), it will just get saved “as is”.
This is where a strategy is helpful in the form of a plugin called Tasty Pins.