To speed up DNS most name servers on the internet will cache (remember) DNS records themselves, so that they do not have to look up records each time they require them.
When you visit a website, it is probable that your ISP's name servers will remember the IP Address, so that the next time you (or someone else) requests that website, the IP address can be retrieved more quickly.
In this instance your ISP's name server is using a cached DNS record. It will use this cached record for a period of time, after which it will retrieve the record again. It will again cache the record and the cycle will repeat.
This is why when you make changes to your DNS some people will not see the change immediately as they may well be seeing a cached record.