The slider can only step by + - 1 increments. A cool feature would be to add a lv_slider_set_step function that change the increment to a user value.
There is already a step for the spinbox with:
As a side note I have to say that I am unsure how it works on the spinbox. Maybe I understand it wrong? When I try the online python simulator using the code from the spinbox doc page and add the spinbox.set_step(2), the step is only changing for one of the digits.