She could be in civvies (I, for one, would love to see her goth look brought back when she's not in costume). Or Cable is traveling back to the NXM era. Or Liefeld just looked up X-23 for reference and used something from the KYost NXM. Remember, Land used that look for her cameo panel in Weapon X #1.
Just because that's what is on the cover doesn't necessarily mean anything.