how do people get with the RX 570 and RX 580 - 37 - 39mhs ?? Very mongo video on the Internet, and many of them look very believable

Probably used the benchmark setting in Claymore, set it to an earlier epoch and it will look like it's mining faster.
"-benchmark benchmark mode, specify "-benchmark 1" to see hashrate for your hardware. You can also specify epoch number for benchmark, for example, "-benchmark 110"."