Maybe with extra space for the future they wont have to adapt legacy products that dont accept a bigger battery like they did for the 9.0 and now the 12.0.
well, it is possible that they are allowing for expansion inside the case, but the battery seems to be getting pretty big as it is. i would imagine that while it is not a big deal on a stationary saw, tower/hanging light, or a lawn mower, it seems to me that it is getting pretty big and awkward for a hand tool.
besides, if they wanted to future proof the tools, they could always ship a new cover that is larger than the current battery, rather than one sized to fit it. if they did that with the 9AH batteries, they could have designed them for the 12AH's to have fit.
were i them, i would standardize on several general "sizes" (say, three for M18, and two for M12), and upgrading them to higher AH's using better cells.
of course, i am not the one who has grown a billion dollar tool company, so my advice is not necessarily worth a lot . . . .