Most weight stacks themselves are fairly accurate – the discrepancy presents itself in terms of mechanical advantage.
Mechanical advantage is a concept which encapsulates the purpose of machines in the first place – that they can move more resistance than the force applied.
In other words, there is not a 1:1 ratio of resistance to effort when using a machine, due to the weight-relieving mechanics of pulley and lever systems.
With a little bit of physics knowledge and some basic math, you can actually calculate the cable and pulley ratio of resistance per machine – check this article out for more.