Name servers and load balancing
When GTM is load balancing, it computes a new assignment of nameservers to data centers as often as once per minute. There are approximately 6 million active nameservers on the Internet, but most of them have only insignificant demand behind them. To bound the computation required by load balancing, GTM only balances those nameservers that are estimated to represent a substantial portion (95 percent by default, but this is configurable) of the demand on a domain. For most domains with large user bases, this typically runs between 20,000 and 100,000 nameservers. This means that some users, those behind nameservers representing very small demand, can get mapped to a data center even if the data center is considered to be over its load target.