Hit highlighted summary

Haven’t you ever wondered why for some results, the summary does not contain any highlighted word?

Although the found document file does contain the searched word.

After having spent sometime, I think I found the explanation.

Basically, it is explained at http://goo.gl/RiSFBx.

In short, SharePoint keeps only 10,000 characters for each indexed file for the summary, and highlights the searched word only if it appears in the saved text.