Why does thrust rating affect Vr
Our FMC appears to automatically calculate v-speeds for full takeoff thrust for an initial calculation. After using our computer to calculate v-speeds using TO2, I noticed that while V2 was within 1 knot of the FMC calculation, Vr was about 6 knots different between the FMC and computer. That is until TO2 was selected in the FMC, then the FMC Vr changed significantly to be within 1 knot of the computer calculation.
Why such a big difference in Vr based on thrust setting.