TRichView.com

As far as I understand, adjacent items with same style number will be merged unless the style has rvprConcateProtect property in Protect.

However, after inserting several items into TRichViewEditor by Add() function, I found there are some adjacent items stills remains separate while they have the same style number.

Can anyone give me a hint?

Only editing-style methods merge text items when possible.
Viewer-style methods, including Add(), do not do it.
You can use NormalizeRichView function (RVNormalize.pas, included in RichViewActions), it merges similar text items and fixes some errors in document structure.