I don`t know. I kinda looked at it from this pov:
What`s an am2 ? 940 pin ? or some silly amount of pins over the 939 ?
It`s usually the case that they make changes to a piece of kit, termed "advancements", so that it preceeds a tidle-wave of upgrades u need to make alongside them. In alot of cases unnecessarily, as a bad example a pci-e i/o card. Soon enough the pci version will no longer be made.
In many instances, u look at the pin-outs of something like a 939 chip and find 30 pins doing the same thing, 50 reserved, and so on. So why jump to 940 ? Other than an "advancement". Similarly the 940 would likely or unlikely actually use less pins, obviously depending on design.
The arguement of "so the user can`t mistakenly put the chip in the wrong mobo" doesn`t wash anymore either. Last time I checked, the 939 DDR1 mobos has 4 blank pin placers.. well u just change the orientation surely ?
Chances are, like with the re-introduction of cartridge based cpus (that u could have noticed had less connections to the mobo than the chip had to it

), a new manager comes into an area of design. They want to make changes, save stuff, appear to be good for the company by being radical - for arguements sake, let`s suppose a new dood saw the axing of the 939 machinary and said "Hoooowwwlllld on a moment". - and saved the company something.
pfft who knows. It`ll be a generation before they`re more eco-nomical with their construction process vS new $$$ than they wish to be atm.