It's because Perseus was the son of Zeus according to Greek mythology. I haven't even seen any of the Clash of the Titans movies yet and I even know that.
Percy Jackson is obviously a fictional character that was named similar to Perseus, but was the son of Poseidon.
Rest of my Top 25 Leighton Meester ~ Emmy Rossum ~ Shantel VanSanten ~ Caitlin O'Connor ~ Audrey Nicole Kristin Kreuk ~ Erica Durance ~ Allison Mack ~ Sara Jean Underwood ~ Sydney Barlette Tierra Lee ~ Jennifer Aniston ~ Kate Beckinsale ~ Giada De Laurentiis ~ Laura Vandervoort
Honorable Mentions Sarah Lancaster~Jenna Fischer~Cobie Smulders~Jana Kramer~Deborah Ann Woll Elisha Cuthbert~Minka Kelly~Cheryl Burke~Moon Bloodgood~Kate Upton~Holly Madison Hayden Panettiere~Julianne Hough~Zooey Deschanel~Lyndsy Fonseca~Beth Behrs Olivia Wilde~Sarah Shahi~Candace Bailey~Stana Katic~Rosie Huntington-Whiteley
Don't anyone laugh, but I really thought that Percy Jackson though based on the Greek Mythology at least followed the important 'relationships', ie. they didn't make Hades rule the Seas, Hades rules the 'skies' and have Poseidon head poncho from his castle in Hell.