The Vr-V2 relationship is pretty much a direct function of the available thrust (or the T/W). If one or the other is constrained by other criteria (whether it be the V2min of 1.13Vsr or some other value) then that will stay fixed, and the other of the pair will have to adjust accordingly.
So if at a given weight your V2 is essentially already at V2min, then it can't go down, only up. But with less available thrust your acceleration from Vr to V2 will be less, so to ensure you achieve the minimum V2 at the required height, you need to increase Vr a bit so that the lower acceleration is compensated by starting the rotation a bit higher in speed.
That then opens up the possibility (but not a necessity) to increase V1, since Vr is always an upper bound on V1. As FD says, increasing v1 improves GO performance (at the expense of worsening the STOP case) so with less (assumed) thrust the GO case is made worse while the STOP case is hardly affected. therefore you can "rebalance" the GO and STOP cases by shifting V1 up a bit to try to get a better overall distance.