If you calculate it back to MNodes/GHz per Core you will get pretty the same numbers... but the Intel Core 2 Duo has a nice performance!!
AMD 64 3800 X2 (2GHz) has two cores --> 13 MNodes/GHz/Core (=52MNodes/(2GHz*2Cores))
Intel P4 2,8 GHz has one core --> 12,2 MNodes/GHz/Core (=34MNodes/(2,8GHz*1Core))
Intel Core 2 Duo (1,6 GHz) has two cores --> 15,7 MNodes/GHz/Core (=52MNodes/(1,6GHz*2Cores))