Thank you for the response.
Yes, search configuration are all correct and I did not tokenize any fields for cms.file page types.
I did copy all the content from abc.xlsx to xyz.xlsx file and tried the smart search again. This time when I search from "Child" it did not show the xyz file but again showed abc file, which is very weird.
There is no difference in abc and xyz file, except abc was created a very long time back (6 months old).
I am really surprised what could be the problem.
Thanks in advanced.