The member number is like giving someone's account a reference code. Your computer and the server doesnt recognise you by your user name all that often, but mainly by your number.
When you sign up you get a number, which is usually just the #th person who has joined, so I can say I was the 662nd person to join. But in reality, if a server has some issues with counting, it can leave a few blank spaces, so numbers that dont correspond to a member. So I may be number 662, but I am only the 650th member. That was my theory, and I have administered a forum that had that exact problem.
The front page member count shows you how many accounts there are, not what number the count is up to, so it is correct. But since csweb joined before there were 1000 members you cant say he is the 1000th member, but he is the member with number 1000.
But if Darren says he's number 1000, I guess he is