Confusion about the meaning a parameter
In an analog system there is a sensor, and then some way to convert it to volts. Observe the measurement range in g's

±2g which is the input and 0 to 5V which is the output. Somehow they convert the input to the output.

So what they are really doing is taking ±2g and mulitplying it by 1.25(V/g) which would give you ±2.5V (if you look at the units they cancel out and leave you with volts.

They then add 2.5V to the ±2.5V range, which gives -2.5+2.5= 0V and +2.5+2.5=5V
to get the 0V to 5V range.

The analog system can be represented by a gain, and then an offset, which is a common format in analog systems. No gain is perfect, however (due to resistance mismatches and other sources of constant manufacturing error.) So the 1.25 can be ±1% or 0.99 or 1.01 of 1.25. So one unit could have a 1.25*0.99= 1.2375(v/g) gain and another unit could be 1.25*1.01= 1.2625(v/g) 

This means your knowledge of the original signal will vary by 1% without calibration. If the scale factor is calibrated, then the scale factor tolerance will not matter and it can be multiplied out.