It is often the case that for a period a user can access both his old and new ISPs' systems. Only the username and password details entered determine which.
The fact that there is sync with the exchange doesn't really mean much except on an new connection rather than a migration.
We even get people complaining here that they are still on their old ISP hours after they expected to have migrated. Simply swapping their login details (or router) puts them on the new one. Swapping them back reverts them to a working connection on the old one.
I believe it is to do with the updating of the various BT databases having an overlap period for the routing to the two ISPs' radius servers.
My broadband basic info/help site - www.robertos.me.uk
| Domains,website and mail hosting - Tsohost
Connection - Plusnet
Extra Fibre (FTTC). Sync ~ 54.0/14.9Mbps @ 600m. - BQM
"Where talent is a dwarf, self-esteem is a giant."
- Jean-Antoine Petit-Senn.
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.