Not sure what you're talking about. See airbnb.com's SOA handing out different serials on their dual DNS Dyn and NS1:

→ ~ » dig +short soa airbnb.com @ns1.p74.dynect.net. ns1.p74.dynect.net. hostmaster.dyn.com. 15932 3600 600 604800 1800 → ~ » dig +short soa airbnb.com @dns1.p08.nsone.net. dns1.p08.nsone.net. hostmaster.nsone.net. 1626078608 43200 7200 1209600 3600