For compatibility with Atlas, who have used Code 83 rather than Code 70 for their premium track components for years. This represents a protoype rail height of just under 7 1/4" in HO. (0.083 x 87 = 7.221). Code 70 would represent 6.1" in HO (5.3" in 1:76 OO).
According to this link, average new rail height in North America for 115 to 140 lb rail ranges from 6 1/4" to 7 5/16", so Code 83 is closer to protoype.