Sorry to have kept that one short. But three questions three easy answers.
- Old System is too old
- TwitchNotify sends the “brand new sub” notification always. Resubs are always by USERNOTICE.
- Long and short of it is, that subscriber events are copied from the host channel to the hosting channels. So for a while (until recent changes) you did get resubs too. But of late it’s just new subs.