How DNS test RTT works for getting all nameservers’ RTT of a zone?

When I read 《DNS and BIND》 CHOOSING BETWEEN AUTHORITATIVE NAMESERVERS:

Some of the card-carrying Mensa members in our reading audience may be wondering how the nameserver that receives the recursive query chooses among the nameservers authoritative for the zone. For example, we said that there are 13 root nameservers on the Internet today. Does the nameserver simply query the one that appears first in the referral? Does it choose randomly?

BIND nameservers use a metric called roundtrip time, or RTT, to choose among nameservers authoritative for the same zone. Roundtrip time is a measurement of how long a remote nameserver takes to respond to queries. Each time a BIND nameserver sends a query to a remote nameserver, it starts an internal stopwatch. When it receives a response, it stops the stopwatch and makes a note of how long that remote nameserver took to respond. When the nameserver must choose which of a group of authoritative nameservers to query, it simply chooses the one with the lowest roundtrip time.

Before a BIND nameserver has queried a nameserver, it gives it a random roundtrip time value lower than any real-world value. This ensures that the BIND nameserver queries all nameservers authoritative for a given zone in a random order before playing favorites.

On the whole, this simple but elegant algorithm allows BIND nameservers to “lock on” to the closest nameservers quickly and without the overhead of an out-of-band mechanism to measure performance.

this paragraph I don’t understand,

Before a BIND nameserver has queried a nameserver, it gives it a random roundtrip time value lower than any real-world value. This ensures that the BIND nameserver queries all nameservers authoritative for a given zone in a random order before playing favorites.

why there say gives it a random roundtrip time value lower than any real-world value then the nameserver will queries all nameservers authoritative for a given zone in a random order before playing favorites?


My guess is that if the given low RTT time reach it will send DNS query to the next, and after all, it get all nameservers’ RTT for saving, right?