Q: Keyword counts don't match

When I use the Keyword Manager, Go into Manage Keywords,  select a keyword, and then push the - icon to delete the keyword, Photos asks "Are you sure you want to remove the selected keyword", and also says "n photos will have keywords removed", where n is a number.

Now if I cancel that removal, and go to Search, and enter the same keyword, I see a result saying there are m photos with that number.

But m is usually less than n, significantly less in some cases.

Is my question clear?

Any ideas why?