I'd say it's meant mainly to keep up with changes done outside of router. So there's a hostname you don't control, you add it in address list and if its target address changes, router will update it, after TTL of record in address list expires. Yours can be viewed as a special case, but it still works like this, apparently it doesn't get any special handling. When you added forum.mikrotik.com to address list, its TTL was X seconds. Then when you added new static DNS entry, address list doesn't care, because the address it has is still valid for "X minus how long it took you to add static entry" seconds. If you wait for that long, it will be correctly updated.
People who quote full posts should be spanked with ethernet cable. Some exceptions for multi-topic threads may apply.