The successful selection response will return the article numbers of the first and last articles in the group, and an estimate of the number of articles on file in the group. It is not necessary that the estimate be correct, although that is helpful; it must only be equal to or larger than the actual number of articles on file. (Some implementations will actually count the number of articles on file. Others will just subtract first article number from last to get an estimate.)
We use the estimate of number of articles based on the difference
between the first article number and last article number. So for example, until the first article number gets moved out (meaning the entire discussion), the number returned will look over inflated.