这些数词前被some，several，a few等词修饰时，两种用法均可。可能是为了语句简练，这些不具体的数字当作具体数字用。a few hundred policemen=a few hundreds of policemen几百名警察。
英语中，像hundred， thousand， million， billion这些数词，可用来表示“具体”的数字，也可以用来表示“大概”(不确切)的数字。例如：三百人：three hundred people；数百人：hundreds of people。
表示具体数目，之前加one (a)， two， three， seven等具体数字：one (two， three， seven) hundred people，之后不带of。
若表示“大概”(不确切)的数字，数词本身用复数形式，后跟of，其前不能有具体的数字修饰，例如：
hundreds of workers
thousands of cows
tens of thousands of trees
但是，这些数词前被 some， several， a few 等词修饰时，两种用法均可。可能是为了语句简练，这些不具体的数字当作具体数字用
some million trees = some millions of trees几百万棵树
相关比较：
Several hundred workers were waiting outside the factory. (正)
Several hundreds workers were waiting outside the factory.(误)
Several hundred of workers were waiting outside the factory.(误)
若改为：
Several hundred of the workers were waiting outside the factory.(正)
Several hundreds of workers were waiting outside the factory.(正)
分析：
Several hundred of workers were waiting outside the factory.(误)
该句之所以错，是因为中心词是workers，而Several hundred of是修饰语，多了一个of。
Several hundred of the workers were waiting outside the factory.(正)
该句之所以对，是因为中心词是Several hundred(数词可以作主语)，而of the workers是定语，修饰Several hundred。既然作定语，那么workers前应加the表示特指。
Several hundred of the workers意为：那些工人中的数百人
