Mysterious DNS Problem

[MM]

Well, at least it is mysterious to a rookie like me.

I have registered 3 DNS servers with the internic. However, some ISPs have
the third DNS addresses in their cache, which point to a mirror site. I
waited until the cache was cleared (TTL value expired), and when I queried
the ISPs name server, the #3 DNS value was there again.

Why does it not cache DNS1 or DNS2? Even though I know they were not down?

For example the A record for WWW.MYDOMAIN.COM is 255.255.255.1 on DNS1 and
DNS2. The A record for WWW.MYDOMAIN.COM for DNS3 is 255.255.255.2.

If you were to use home.net's DNS server, you will be sent to 255.255.255.2.
Why doesnt it cache the first value?

Thanks,

Mo