Basicaly there are three ways, to tell how long is requested file: To say it by Content-Lenght that file has X bytes size (good for 3.96%), to say nothing, or just say server does not know by Transfer-Encoding: chunked (bad for 3.62%).
It has almost no meaning, it is same as all my google results, that are below 5%. Some small connection is there. Logically – webmasters, who take care about their websites, usualy wants to have the best website. So maybe this is only evaluation of webmaster perfectionism.
“Transfer-Encoding: chunked” header is in Youtube.com – that should move it up. But it is also in websites with WordPress as content generator. To be honest – there is no right answer whether this header is good or bad.