Zerigo states on their site
When you make a change to your DNS records, it takes a moment for them to replicate to all of the DNS servers. Until this happens, nobody can see the change.
Zerigo's proprietary synchronization engine ensures this happens in mere seconds. We won't name names (to protect the guilty), but many providers take hours to perform updates. In our opinion, that's crazy and it's why we designed our system for much faster updates.
Are they really stating that their service is different from how DNS operates normally, or is this just stating that when you make a change they make sure it instantly removes any old cached records from their systems but doesn't do anything about other DNS servers?