Actually, they can go to 107. The number indicates which server that your board is located on. However, due to recent changes, this number is no longer required to be entered to go to the site. However it is still good to know.
You can only do the specific server number assigned to that forum, or no number at all. Attempting to use a different number will send you to the Proboards homepage.
As I said, due to the recent changes, they don't matter anymore. However, before the changes, if you entered the wrong server number, or did not enter one at all, you would be sent to the proboards homepage.
I don't know why they changed, but I have a feeling much of it was related to the major outage that occured a few months back.
No forum on proboards is allowed to have the same name. There isn't another "nintendoaddict" on proboards. So, I assume that it ignores the incorrect server, and puts you on the server with the one and only Nintendo Addict. But without the server number.