The idea - that it is too expensive for Google to present an accurate accounting of the number of search results on every page except the last one - is ridiculous on its face.
The idea that they are being lazy on the first several pages but then only on the last one give you the real number of search results, that doesn't make any sense. How would it know what the last page was? The answer is that they do know. This is not hard.
The amount of resources available to Google is simply mind-boggling, so any suggestion that they don't know what is going on with every pixel of their search results page is just as mind-boggling. Of course they know what is going on.
Google's search results are fantastic, because how often do you need to go past the first page to get what you were looking for? Almost never. Which is incredible when you think about it. It is a very useful service.
But the misleading number of results it presents on each page except for the last one is deliberate and underhanded. The sole purpose is to make it seem like Google has a huge database of information that it is willing to rank and then share - in the form of links - with the user. Most people fall for it because they don't get past the first several pages of results.
The truth is that Google does have a huge database of information, but they're only willing to share a small part of that. So you get the impression that they have hundreds of thousands or millions of results to present to you, even though they absolutely do not. They might catalog all that, but it's not for you.
It is intentional and deceptive, and it's not hard to understand.