That's the difference that SureFire's R&D have made over the years in bulb/reflector design technology since the P90/P91 was produced.
I was once told that the MN10/MN11 bulbs are about the best possible - perhaps the most advanced bulbs ever created.
PK was not the one who told me this.
It may have to do with the P90/91 having to fit in the smaller space available in the p-series lights. The M3 bulbs are much larger than the P9xs. Once designers are free of a design constraint like size, a performance goal can be reached with fewer compromizes.