Grey’s Anatomy: How can Lexie Grey be an intern only a year after Meredith was?

I just finished Grey’s Anatomy season 3 and obviously it states that Lexie Grey is an intern the next season. I was just wondering how that could be possible since she would have to be 5 years younger than Meredith since Ellis didn’t leave Thatcher until Meredith was 5. That would put them only one year apart. Anyway, I was just wondering so any information would be great, I am very curious.
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