That which is more fundamental to the problem is the unavoidable (at
room temperature) noise from the resistors. Even a "perfect" resistor
with zero tempco has noise, so if you use resistors to measure
current with a high-precision voltmeter, eventually you reach a
resolution where the noise becomes dominant. If you reduce the
bandwidth by averaging, the precision should reach the basic
stability of the resistor - but the (in)stability itself may be
viewed as noise of very low frequency - always too low to filter out
unless you have a very long time.
The resistors chosen for DMM current ranges may just be rational
choices and compromises for the types of performance to be expected
under normal usage and conditions, and considering the noise limits
to resolution, versus the cost of extremely low tempco sample resistors.
Ed
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