There are myriad technical reasons why there is one version of a phone running on one type of chip and another variant of it powered by some other processor. The Samsung Galaxy S4 is a good example of it as its US variant will be powered by Qualcomm Snapdragon 600 chipset instead of Exynos 5.
However, Samsung confirmed us that Exynos 5 does fully supports 20 LTE bands, so we are guessing there might be other factors that prevented the Korean manufacture from launching the Octa-core Galaxy S4 in US. Or Samsung could just be short of Exynos 5 chips for now.