I was enlightened by a former test pilot - apparently, the VSI on SOME of the displays ( we're talking about the altitude tape & VSI indicators being juxtiposed variety ) have thwe VSI scale graduations displayed/placed in a non-linear - logarithmic-like manner - IE the scale distance from 0 to 1 is longer than from 2 to 3 etc. Which allows them to effectively be used together in a predictive fashion.
Thanks for your input