Hot/High
An example of the difference altitude can make was demonstrated to us when we carried out a wet lease for Avianca in Bogota (8,455ft IIRC). Normal Vr would have been 120 -130 Kts. It still was as the Indicated Air Speed, but actual Ground Speed at rotate was, on average, something in the order of 195 Kts. Approach power (B757) is usually 1.16 EPR. At BOG it was 1.26.