"Changing the Document Unit changes the relationship between the width/height and viewBox such that the initial user unit (aka 'px') is equal to the Document Unit. "
"The authors of the SVG specification solved this problem by introducing an abstract “dimensionless” unit called user units. The SVG1.1 specification 1 is quite clear about their definition: 'One px unit is defined to be equal to one user unit.' ... So whenever you read 'user unit', think 'pixel'."
(TODO — read especially the part about the "user coordinate system", that seems especially important)
"Without further specification, one user unit equals one screen unit. To explicitly change this behavior, there are several possibilities in SVG. ... "