I-20的正常时间是60个月？为什么？

yilifang

我申请的是master，但学校I-20上写the student is expected to report to the school no later than 01/14/2009 and complete studies not later than 01/01/2011. the normal length of study is 60 months.

按道理不是2年时间么？有谁遇到这种问题了，15号签证，急