Hi Peter,
This usually happens when a custom code is being used either to change some query arguments or other aspects of the results list.
There are other special cases too, when the plugin changes the final results list count during the post-process, which obviously is not known up until the very last batch of results is processed – so there is a minor difference in the intially anticipated and the actualy results list.
The results are definitely correct, you don’t need to worry about that, it is only the counter at fault. If you happen to use any custom code in relation to the plugin or the search in general, that can explain the minor difference of the anticipaged and the actual results count.