hmm to be honest i think this whole battery market is a mess.
when i started with AA batteries you could buy rechargeable ones with 650mAh now (nearly 20 year later) you can get 3000mAh batteries. But the technologie changed from NiCd to NiFe and NiMH to nowadays LSD NiMH batteries.
so with new technologies and some time these amount of increase is understandable.

but if it comes to Li-Ion or LiPoly then i think differently. The higher capacities had been quite expensive at the beginning so companies used to sell 750mAh, 850mAh or even 1000mAh batteries without letting the customers know what they get. They simply used their own format and put their own label, with a strange name, on the battery.
especially Nokia is such a company. they produce several different mobiles with different battery types. and after a while they switch back to old known types. but now they can't just rename them and sell them with more mAh. the new batteries could be used in the old devices as well.
so they still sell the old types with the old specs, even if there are batteries with the same diameter and more mAh.

if i'm wrong show me the truth, but so far that is what i think. and i can't blame Nokia for it. if i had old products on stock i would try to sell them, as well.

on the other hand. i would always compare batteries to similar ones.
my Huawei e5830 has a huawei battery with similar diameters but an capacity of 1500mAh. so i would say that there is a possibility for 1500mAh batteries that could be used in the N900... but more then that.... nope not at the moment.