What does "you're less than 2 years older" mean?

Not sure if you guys have seen this new show but I was watching The Village season 1 episode 6 on NBC and there's this scene where Katie and Liam are having a conversation somewhere under a bridge and Katie said "I get that no one wants to hook up with a pregnant high school girl..." and Liam said... show more Not sure if you guys have seen this new show but I was watching The Village season 1 episode 6 on NBC and there's this scene where Katie and Liam are having a conversation somewhere under a bridge and Katie said "I get that no one wants to hook up with a pregnant high school girl..." and Liam said "You're in high school" Katie said "You're less than 2 years older" Katie is 17 and Liam is 19. Why didn't she just say "You're only 2 years older"? It was kinda confusing.
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