Zurb Foundation: why can't I redefine a variable in SASS with the desired value in pixels?

Question

css

I'm trying to override the $grid-padding-gutters and $grid-container-padding variables, giving them 20px each, but they still convert to rems and the paddings get smaller than 20px. The values ​​change but not exactly, as I understand it, due to the fact that everything goes into rem's.
Is it possible to get exact values? Or all the same it is necessary to be played with rem?