identical rechargeable batteries
No such thing, every battery will be unique in both capacity and therefore charge/discharge memory, etc. Subsequent charging/discharging cycles will further complicate this. In general, entropy increases.
4 batteries may be made in the same batch and all pass the same tests to be sold as "2700mAh" but they'll probably be, at a guess, a 5% variance in those capacities (i.e. some may be as high has 2900mAh, it's unlikely they're mis-sold as lower than their stated capacity). Batteries intended to be 2700mAh but fail the appropriate test (say only 2500mAh) just get packaged and sold as that lower capacity, better than throwing it away or picking it apart to start again.
In a simplificiation, a set of batteries runs until the weakest is exhausted, so a 2500mAh battery and a 2900mAh battery would act like two 2500mAh batteries, which means you're charging the 2900mAh when it isn't fully discharged which, over time, will change it's characteristics.
Best thing to do, if you really are anal about it, is to charge batteries with a clever charger like the Technoline BL-700 which will give you an indication of the capacity of an individual cell. You can then pair up cells with similar capacities, and throw away the ones that eventually become duff (I've had a few 2200mAh cells that started off OK but quickly dropped to less than 1000mAh and were more trouble than they were worth).
