Is this the rounding of the ui?
It looks like it but i get no density when i type this value, but it works with 0.001 for example
i can confirm, when setting a driver with a low value, it does in fact use a rounded version of the source value. attached are screengrabs of the x location driving the z location on an object. checking in the console you can see that the driven value is in fact rounded down to actual zero.
edit: to make the issue even more explicit, here you can see where i have two objects whose x locations should be the same via drivers, but at small values, they fall out of alignment.