I'm suprisede Sian is ranked top 500 in juniors as she hasn;t had too much success. She should be much higher though, she just doesn't seem to have got the itf results which her national results would indicate.
That is because a huge chunk of Sian's points comes from doubles! So far she's won doubles titles in two G4s and a G5, plus has reached the final of three other tournaments.
She's also a tad overranked at the moment with points from South Africa and Bangladesh, although I'm fine with people giving their career a kick start with some cheap points!