Omnitrix aliens are created to be the best example of the species, not copies of the DNA donor. I think they're like athletes myself, which is they XLR8's posture is more theropod-like.

Being the peak form of the species Jen is able to reach speeds nearing 600 mph. However she's been training Domino get past 370.

As Kinecelerans both XLR8 and Domino are partially bio-cybernetic. This allows the species to reach speeds far beyond their ancestors. However the faster one moves the more their posture changes from vertical to horizontal. Which is why Domino stands upright.