Parent or Child NS records ?

[ddd]

Question; I see that our parent (.gov - or any of the root-servers)
hands out 3 NS (& glue A records) for our domain, but another BIND
server has different results at different times...the one set which
contains auth00 is held on on parent, but the set of 3 which are all
151.200.x.x are held on our own DNS server, NS. Why the difference at
different times ?

Non-authoritative answer (from d.root...)
uspto.gov       nameserver = NS.uspto.gov
uspto.gov       nameserver = AUTH00.NS.UU.NET
uspto.gov       nameserver = NS2.uspto.gov

NS.uspto.gov    internet address = 151.200.97.38
AUTH00.NS.UU.NET        internet address = 198.6.1.65
NS2.uspto.gov   internet address = 151.200.97.39

but when I check the DNS server that I'm dialed into at time-1:
> uspto.gov.
Server:  [207.69.188.186]
Address:  207.69.188.186

------------
Got answer:
    HEADER:
        opcode = QUERY, id = 7, rcode = NOERROR
        header flags:  response, want recursion, recursio
        questions = 1,  answers = 3,  authority records =

    QUESTIONS:
        uspto.gov, type = NS, class = IN
    ANSWERS:
    ->  uspto.gov
        nameserver = mail1.uspto.gov
        ttl = 1065 (17 mins 45 secs)
    ->  uspto.gov
        nameserver = mail2.uspto.gov
        ttl = 1065 (17 mins 45 secs)
    ->  uspto.gov
        nameserver = ns.uspto.gov
        ttl = 1065 (17 mins 45 secs)
    ADDITIONAL RECORDS:
    ->  mail1.uspto.gov
        internet address = 151.200.97.38
        ttl = 1065 (17 mins 45 secs)
    ->  mail2.uspto.gov
        internet address = 151.200.97.39
        ttl = 1065 (17 mins 45 secs)
    ->  ns.uspto.gov
        internet address = 151.200.97.38
        ttl = 83865 (23 hours 17 mins 45 secs)

But later at time-2:
> uspto.gov.
Server:  [207.69.188.186]
Address:  207.69.188.186

------------
Got answer:
    HEADER:
        opcode = QUERY, id = 10, rcode = NOERROR
        header flags:  response, want recursion, recursion
        questions = 1,  answers = 3,  authority records =

    QUESTIONS:
        uspto.gov, type = NS, class = IN
    ANSWERS:
    ->  uspto.gov
        nameserver = NS2.uspto.gov
        ttl = 86321 (23 hours 58 mins 41 secs)
    ->  uspto.gov
        nameserver = NS.uspto.gov
        ttl = 86321 (23 hours 58 mins 41 secs)
    ->  uspto.gov
        nameserver = AUTH00.NS.UU.NET
        ttl = 86321 (23 hours 58 mins 41 secs)
    ADDITIONAL RECORDS:
    ->  NS2.uspto.gov
        internet address = 151.200.97.39
        ttl = 80513 (22 hours 21 mins 53 secs)
    ->  NS.uspto.gov
        internet address = 151.200.97.38
        ttl = 80513 (22 hours 21 mins 53 secs)
    ->  AUTH00.NS.UU.NET
        internet address = 198.6.1.65
        ttl = 158110 (1 day 19 hours 55 mins 10 secs)