F6 China taxation -2008 书中例题的一些疑问

书是在东华大学买的，　有些例题使我非常疑惑．

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Q: A manufacturer appoints the agent to sell 20,000items for RMB80 each,commission RMB5.00 each; the agent sold all the items at RMB90 each

A: Business tax =[(90-80)*20,000+5*20,000]*5%=rmb15,000

    VAT =(90*20,000-80*20,000)*17%=RMB34,000

对这个答案我感到非常疑惑，进销差价已经缴纳ＶＡＴ　了，怎么还要交纳ＢＴ　呢？　

小的学艺不精，请高手帮忙解答，谢谢！