They don't.
Just for you, I pulled seven (7) AA cells out of a fresh pack from the
store, taped them down to my desk, collected a few other things,
and made these measurements:
AA Cell Open at 25 mA Calc int R, Ω
#1 1.596 1.546 3.285
#2 1.601 1.551 3.285
#3 1.598 1.539 3.876
#4 1.578 1.548 1.971
#5 1.600 1.539 4.008
#6 1.598 1.526 4.730
#7 1.600 1.547 3.482
The (open/unloaded) voltage of the cell is determined by
the cell's "chemistry" ... the actual chemicals and metals
it's constructed from ... that undergo the chemical reaction
to deliver current between the terminals when a conductor
is connected there.
-- The variation in voltage among these 7 cells is about 1.44% .
-- The variation in the internal resistance of these cells ... an
indication of how much current (amps) a cell can deliver ...
varies over a range of almost 140% . I am surprised.