From the about.com article:
Because of limitations inherent in the study of ancient history, we don’t know for certain when many of these early Greek poets were born or died. Some dates, like those for Homer, are guesses. New scholarship could revise these dates. So, this early Greek poets timeline is a way to visualize relative chronology within the same genre. The genres here are: epic, iambic, elegiac, and lyric poetry.
Read the article here.