The speed of a heavier pellet is lower in the same airgun versus a lighter pellet. The only thing that counts is how much time gravity has to do the pulling. It is only because heavy pellets take longer to get to the target that their trajectory is more bowed. A lightweight pellet traveling as slowly as a heavy weight would have an equally bowed trajectory. This means fast pellets lose energy more rapidly than slower pellets.
Energy equals mass times velocity squared.
Since the energy imparted to a pellet is about the same and a pellet doesn't lose mass, it can only make up for the different weight by changing velocity. In fact, fast, light pellets lose energy so rapidly, after 35 yards or so they can be traveling slower than heavy pellets. This is inconsequential in 10 meter target shooting, but it becomes a major disadvantage in hunting and field use. Flat-nosed wadcutters punch perfect holes in paper targets to aid in scoring, and are required in competition. The slight effects of higher air resistance on accuracy due to the flat head are unmeasurable at 10-meter range, but do become noticeable at 35 meters and beyond.