Why Google index different title when search different keywords?

I change my title recently to make it be more user-friendly for Google search, but I find one problem for that: When search keyword A, it shows the whole title that I changed; search keyword B, it shows the site name - like this: abc.com, it shows a b c as title in search results. The keyword ranking is normal.

How to solve this? I had changed it around 15 days. Need some suggestion.

Do you have 3 word domain ?

Yes, it is a 3 words domain.

Many factors - it could be using dynamic search results, that will bypass what you have set. This helps google to list appropriate results on the serp that could be relevant to the searcher's search. Or if you have just changed that info, then it will take time for it to reflect the correct one. There are many google servers, and it will take time to get the info replicated to their servers.