I made a page about this:
Basically, unbalanced bridge sensitivity is non-linear;
for bridges balanced @ 50 Ohms, signal amplitude difference from 0 to 50
is the same as from 50 to 150. Signal change from 500 to 1000 Ohms
is only 10% of change from 0 to 1000 Ohms, so far more sensitive near 50 Ohms,
and "calibrating" with a different LOAD does not help.
Oristo