Why would delegated nameservers ever be different to authoritative nameservers?

If you go on intodns.com and type in stackoverflow.com, the parent server tells me that nameservers for the domain are here:

ns1.serverfault.com.   ['198.252.206.80']   [TTL=172800] 
ns3.serverfault.com.   ['69.59.196.217']   [TTL=172800]  
ns4.serverfault.com.   ['69.59.196.122']   [TTL=172800] 
ns2.serverfault.com.   ['198.252.206.81']   [TTL=172800] 

However, the zone file itself, which is authoritative, tells me that actually the nameservers are here instead:

ns2.serverfault.com  ['198.252.206.81']   [TTL=300] 
ns1.serverfault.com  ['198.252.206.80']   [TTL=300] 
ns3.serverfault.com  ['69.59.196.217']   [TTL=300] 

Can any one explain why ns4.serverfault.com. would be listed as a nameserver when it isn't? What would happen if the resolver went to this address trying to find the A record for stackoverflow.com?

Whoever registered the domain name provided 4 nameservers to the registrar, so the registrar have added 4 glue NS records for the domain with the relevant addresses.

For some reason, which only stackoverflow will know, only 3 nameservers have been added to the zone file. While ideally the nameservers listed on a domain's registration (as visible in whois) should match what's in the zone file, these 2 lists come from different places and so it's possible for them to be different.

It appears that ns4 does actually work so any requests going to that IP address will get a response.