Best guess as to why this happens is that your nerves are on edge the first few times. You don't know what to expect, you're watching the boss very closely because you don't know what they can do, you really want to brag to your friends that you "killed so and so first try".

Even as early as a second try, especially when you got close and have therefore assessed the boss as "easy", you relax a bit and start to think you know what's coming. And that's when you've really got to learn the fight for real.