ClouDNS DNS Failover: Switches IP in 1 Minute—Why Is It So Fast?

I'm testing DNS Failover with ClouDNS.net for my website, and it seems to work:

Main: IP Website 1

Backup: IP Website 2

When IP Website 1 goes down, it takes approximately 1-2 minutes for ClouDNS to switch to the backup IP.

Tested with 2 different ISPs

Note: Users need to close and reopen their browsers to flush the local DNS cache.

The TTL on the A record is set to 1 minute.

However, I don't understand why the switch to the backup IP happens so quickly. I've read everywhere that DNS propagation usually takes around 12 to 72 hours, and that many DNS services ignore TTL settings.

Is there something specific about ClouDNS that enables them to force DNS propagation very quickly to slave servers, compared to traditional DNS services?

Or is it just my ISP's DNS servers refreshing DNS records every minute? And in that case I can also use classic DNS service also ?

Thanks!