This can sometimes happen when there are similar pages that were indexed:
In order to show you the most relevant
results, we have omitted some entries
very similar to the 19 already
displayed. If you like, you can repeat
the search with the omitted results
included.
If you click the link at the bottom of the last page, you should see all 5 pages.
For more information, you can see this book which explains about that feature.
It seems like this is also for optimization reasons. From Google's page:
In addition, when you click on the
next page of search results, the total
number of search results can change.
In this case, we realize that some of
the query results are duplicates, and
collapse those duplicates so that you
can find the specific result you're
looking for more easily. Collapsing
the duplicates decreases the estimated
number of results, as well as the
overall number of results pages.
(emphasis added)
Best Answer
If you go to https://www.google.com/preferences?hl=en-GB#languages you can set it up the way you wish: