They might be using Afilias Managed DNS:
Afilias Launches Managed DNS Service
Other domains use it, etc., e.g. aig[.]com
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As for using a variety of TLDs for the nameservers, that's a common strategy of DNS providers, in order to have redundancy in case an entire TLD goes down. (i.e. if .com itself ever went down, then one wouldn't be able to convert a nameserver address under .com into an IP address, etc.).
EasyDNS does this, for their customers. e.g. for School.com:
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the nameservers are:
Name Server: dns3.easydns.org
Name Server: dns1.easydns.com
Name Server: dns2.easydns.net
Name Server: dns4.easydns.info
This is discussed in Mark Jeftovic's book, "Managing Mission - Critical Domains and DNS"
Managing Mission - Critical Domains and DNS
as "TLD redundancy." (Mark is the CEO of EasyDNS)
Even Goldman Sachs does this for gs[.]com:
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Name Server: ns7.gs.com
Name Server: ns2.gs.com
Name Server: ns6.gs.com
Name Server: ns1.goldmansachs.info
Name Server: ns4.gs.com
Name Server: ns1.gs360.net
Name Server: ns1.gs.com
Name Server: ns3.gs.com
Name Server: ns5.gs.com
Name Server: ns1.goldmansachs.biz
As does Amazon for amazon[.]com:
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Name Server: ns2.amzndns.com
Name Server: ns1.amzndns.net
Name Server: ns1.amzndns.com
Name Server: ns2.amzndns.org
Name Server: ns2.amzndns.co.uk
Name Server: ns1.amzndns.org
Name Server: ns1.amzndns.co.uk
Name Server: ns2.amzndns.net
Apple, Google and Facebook don't, though.
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