Comment count for individuals wrong

When a person leaves a comment, the number of comments that they have made, (displayed beside their name), is sometimes wrong. It is not incorrect for all commentators, just some. For example one person has left 204 comments and that is correctly displayed beside her name. Another person has left 175 comments but the number beside his name is “3”. Others seem to be stuck at 15 even though their actual number of comments is much higher. A plugin that I use, Top Commentators, (when set to all time), displays everyone’s count correctly. I have looked at the comment table in the database and everything is OK there.

So, how do I trouble shoot this. When: “Comment by June (29 comments.)” is displayed on a comment, where is WordPress pulling the number from and why might it say “29” when the table shows her as having left 64 comments?

Thanks.