I [see](http://blog.zhuzhaoyuan.com/2009/08/creating-a-hello-world-nginx-module/#comment-1814): 

> the value of content_type_len is usually the same as content_type.len, but in some case, i.e. when there’s charset setting with the Content-Type HTTP header, e.g. given header “Content-Type: text/html; charset=UTF-8″, content_type_len (23) is less than content_type.len (38).