Striking taxes cardio differently. It's more like a marathon, whereas wrestling is a constant power push. So a lot of wrestlers tend to overload on their strikes and have shoulder stiffness, tenseness that drains cardio as well. Resting in those 2 sports is also different. In striking you take breaks by clinching, moving on the outside and resting in between rounds, while in wrestling you lean on the other guy (I'm sure there are a bunch of other tricks as well). Romero wrestled his whole life, he's used to one aspect, but he's a relative rookie in the other and you combine the 2 in MMA, which adds another dimension.