Dont know, but those pickups are the hottest thing since hot sliced bread!
Might be because alnico 5 magnets cut a bit better and create more of a shreddy type tone, not sure though. If so they could maybe rely on the active setup for more fatness and distortion and use the alnico 5 mag for a bit more cut and top end.
Or it could just be that these are meant to be totally different sounding pickups and the different magnest make them what they are and thats what seymour wanted to do
The custom made Active Heavy Metal LiveWire that I have in the bridge on my Jackson Kelly, actually has a ceramic magnet instead of an Alnico-5 magnet. That's actually what's "custom" about the pickup compared to an ordinary LiveWire.