I might be wrong, but I think a key difference is the breed of pig they use, especially for Jamón Ibérico. Maybe it's the black-footed pigs? And the diet for those specific pigs, eating a lot of acorns, is also quite unique, I think. This influences the fat distribution. Teh curing time might be longer too, compared to some other hams, which gives it a different flavor profile. I learned a bit about it when I was backpacking in Spain last year, but I'm no expert, just an observation.