NW is part of Dotster, GoDaddy has thier own backorder service (that sux).
NameCheap is one of the 120 something companies allowed to register names on behalf of people.
NW rents (or however they arange it, possibley per name or commission) NameZeros direct connection so that NW will be able to get names for its clients.
The more registrars one has catching names for them (supposadly) the better chances you would have
SnapNames uses 17 I think. BuyDomains, UltSearch, NW, TheFirstName, DropWizard, DropKing etc. All rent connections
NOTE: this is just what I am aware of, no warranties!