Slight variations on the electronics give slightly different characteristics, and hence different power draws... but also, the batteries are also going to have slightly different capacities, so you have random amounts of power draw clustered around a given value, and random amounts of capacity also clustered, so combined you might end up with a detector drawing 70uA on a 540mAh cell, and one doing 50uA on a 560mAh cell, but the cell voltage they start chirping at is exactly the same.
