Some providers employ BGP anycast for their NS servers.
As a user, when doing a traceroute, many may show an identical location for both servers, for a given client.
When he.net used to anycast their DNS from different locations for a client in Europe, and one of their POPs went bust for a few hours, the
ns server from that pop was unavailable for a number of minutes, prior to being taken over by another POP.
Basically, doesn't it take a certain number of seconds (or even minutes?) in BGP for the takeover to take place? Does it mean that if a provider is always routing all NS to a single location, then at any time, such setup would be completely cut-off from certain clients for some number of seconds should a certain POP go bust?