Posting highlighter not returning proper sentences

I have this chunk of text on which I am searching for specific string and using highlighter option. Everything works fine, but when I use the posting option for highlighting, It doesn't return the sentences as expected but the returned highlighted text is limited by default 100 chars.

Need to know if I am doing anything wrong?

This is a known issue. While other highlighters try to understand the query in order to highlight where the query matched, the postings highlighter just extracts terms from the query and tries to compute a summary of the document that is relevant to the query terms. If you need the highlighter to be more accurate, you need to use another highlighter.