I think it wasn't necessarily *auto-detected* before - I think it had hardcoded values which were actually only valid for most North American carriers. It's become more sophisticated in F12 and handles a much wider range of connection types. While that makes it very slightly more trouble if you happen to have a carrier which uses the hardcoded settings from previous releases, it's a significant win for people on any other carrier
It's not actually technically possible to entirely auto-detect what settings should be used, as all NM can auto-detect is the attributes of the hardware involved. Dan Williams, NM lead developer, goes into more detail on the changes here:
he notes that we can auto-detect most settings for GSM-type networks from the SIM card ID, but obviously that doesn't apply to CDMA-type networks which don't use SIM cards.