I am using Sharepoint 2013 Foundation version.

After search string is input at search field and enter key is pressed, list of search results is shown on search page. At the bottom of the search result page there is text showing 'About 130 results'. As there are only first 10 results shown on each page, I press right arrow to go to next page showing the next 10 results, but on this second page the text at the bottom shows 'About 131 results'. This figure keeps changing to 130, 132, 128, ...etc, as I navigate to subsequent pages. On the last result page, the word 'About' disappears and the figure is 133.

It appears quite strange that the figure keeps changing on different result pages.

May I know how this phenomenon can be understood and interpreted? Is it normal?

Thanks a lot for advice!

As it could take a lot of time to return the complete set of results just to count them, search only returns an estimate. As you drill down deeper into the results it tends to get more accurate. Here's an example from one of my classes: