Siden man har litt interesse for slike ting så ble det å spørre onkel google.
Most likely reason is client side cache is wrong, but a
negative cache entry is only 5 minutes so that would imply
it didn't last long or that you actually had a wrong (but
actual) answer in the cache.
If you eliminate the client side cache it might be due to
the nslookup using a different set of DNS servers (e.g.,
internal vs. external) than the ping (client computer)
resolver.
This comes from mistakenly trying to us two 'sets' of
DNS servers on the client NICs.
Internal Clients MUST use strictly the internal DNS
servers -- and these internal DNS server
must be
responsible for resolving/forwarding any external name
requests.
Kort fortalt, nslookup og ping kan bruke forskjellige dns hvis det er satt opp på pc.