Here's why:
* The lily is a real object: The speaker sees a lily, a white flower, on the woman's forehead. It is not a symbolic representation of something else.
* No comparison is implied: The poem doesn't say the lily is *like* something else or *represents* something else.
* The lily's meaning is ambiguous: While lilies can symbolize purity, innocence, or death, in this case, the poem doesn't explicitly assign a symbolic meaning to the lily.
The line's significance lies in its visual imagery and its potential connection to the poem's overall themes of beauty, deception, and death. The lily's presence, especially on the forehead, could be interpreted as a sign of beauty, fragility, or even foreshadowing the woman's fate.