Getting CS is about much more than just last hitting. In fact, solo laning itself revolves almost strictly around last hitting in high level play. You need to know how to balance all of the following:
Last Hitting: the skill of knowing how much damage you do to a minion to ensure that you obtain the gold from killing it.
Keeping the lane where you want it: This is a huge factor, some champions like Malzahar and Morganna can push harder than champions like Ryze because of their AoE abilities so should the Ryze player wish to keep the lane in the middle he must attack the wave more than just when the last hit is available.
Some champions however excel at killing minions under their own tower and may want to keep the lane there for safety while others have many escapes and can push to the enemy tower all they want and still escape back to their own should the enemy jungler gank. Part of mastering your favorite champions is learning how to balance risk vs reward with your available tool set.
Harassing: You also might not want to only keep an eye on the health bars of the enemy minions, you should also keep an eye on your own. This is especially important in top lane where the enemy usually last hits in melee range. Therefore you know where the enemy will need to be in order to get that gold. If you move or attack in such a way that they cannot reach that low hp minion without taking significant damage they will miss the gold or better. The #1 rule of hunting is: attack your prey when it attacks it's own.
Counters: Most champions have a wide array of other champions that they do particularly well against. Counters usually are formed when a champion excels in one of the above areas that their opponent lacks.