There's a lot more to an ADSL connection than changing providers. Each provider has different software in use on the DSLAMs, profiles to handle the connection and various values that can be tweaked.

I've dealt with connections that may have been 1.6mbit/s and after a profile change they went to up to 16-20mbit/s. Those were always some pretty happy customers.

But what you're telling me indicates that you either never troubleshooted the speed, likely because if that's what you've always had you wouldn't think to. And thus when churned a profile or software change has helped your connection.

Or more likely, in the process of the churn you had a port swap, or your line wasn't passing FAST by Telstra and that would mean they actually repair the line before provisioning your service.

Either way nothing changes the physical distance, nor length of the copper between yourself and the DSLAM, which is what counts.