In particular, one your ISPs/their ISP's DNS servers may be caching a record for longer than it's supposed to and will return incorrect and expired data.
The other possibility is different IP's being returned by a DNS server based on where a query is coming from, eg a CDN. If you're in location A and your ISPs DNS server is in location B, the CDN's DNS server may return a different IP based on if the request is coming from A or B. ECS [0] is supposed to mitigate this, but may or may not be used.