Just my guess:
Since actual temperature hasn't changed, hence density hasn't changed you get airborne at the same speed, so Vr for actual temp is used.
Since with an assumed temp you always have the option to use max (fixed derated) thrust you need to have speeds which correspond to the higher thrust, so again using actual temp for V1 (which has a relation with Vmcg, which relates to actual thrust, which relates to actual temp).
V2 relates to Vmca and Vs, so same story.
Standing by to be corrected.