Well, to accommodate the growing grunge and punk scene, in the mid-late 90's many companies, including Fender, made the pickups hotter. The main problem with most MIM pickups are that they are kind of low-power. The hotter pickups fix that. This was discontinued in I believe 2005, and the pickups went back to how they are nowadays. Not to say modern MIM's are bad, 1996-2005 ones were just better IMO.

my old 95 MIM had really powerfull pickups. In fact when i put a dimarzio fs2 in it, i was suprised at how the volume dropped when it was selected. I dont know if this was design for the time though. Its just my limited experience of 95 mim pickups.