The slower bullet isn't flatter. Whether slow or fast, heavy or light, bullets fall at the same velocity. It's a pretty common phenomenen for slower bullets to hit higher on a target and the reason is that the slower bullet gives the barrel just a tiny bit more time to rise before the bullet exits the barrel.
Not sure I follow this logic. They are both zeroed the same and shot at identical ranges. Pushing the same bullet to faster speeds with the same zero always results in a flatter trajectory with the faster load.
If you don't change the sights and at 'closer' ranges the slower bullet may shoot higher due to the phenomenon you describe. Happens all the time with pistols - if you don't adjust the sights.
