In the episode A scandal in Belgravia (Sherlock season 2 episode 1), when Dr. Watson is blogging and his blog text is shown to the viewers overlaid on the wall of 221B, Baker St., why is the text that comes atop the Bull's Head erased?
The onscreen text interacting with the environment is a stylistic choice and happens more often than you may think. The reason for the text itself is to convey important information in an interesting way, avoiding the tedious and clunky ways exposition is often presented. It also seems to fit with the unorthodox way Sherlock's mind works (seeing things others do not, etc).
Sometimes the text disappears when walked across or in the case of Mary's "Liar" swirls around the two. It can also conform to the shape (like a face) it is mentally projected onto. Once, it was a timer. In the same episode you ask about, the text onscreen is even refected in a window.
It adds depth and weight to the thoughts behind the words, much the way the titles of the film Panic Room seemed to ominously float around the city.