So, I'm a Warrior main and therefor can't speak to much about Ranger. But let me tell you, when I started Raiding years ago, my raid leader used tools to analyze my dps rotation.
He told me I interrupt my auto attacks to much. After me learning not to do it anymore, I improved my dps by like 3k.
Auto attacks hit hard these days, so casting 90% of the Autoattack, only to interrupt it. Negating all the dmg does hurt your total dps.
Also, the guys like snowcrows who pump the big dps numbers, tend to intentionally interrupt their skills to ignore after cast.
Some skills stop dealing damage earlier then the actual cast time is. Knowing that means you can stow your weapon, cancel the unwanted cast time and immediately start the next skill.
Thats however really next level gameplay most of us will never reach.