There aren't any tricks and you aren't seeing what you think. First, you have to realize that HR lags effort by up to 30 seconds or so, so when you "sprint" out of a corner your hr is going to continue to rise even after you are done, and then eventually start to fall again for no apparent change in effort. Second, when you go around a corner you can lose a lot of momentum and spend a lot more effort than you realize getting back up to speed, or even keeping your speed. So again even though you are saying "for the same speed," it's nreally not the same effort, and that's what really matters.

Finally, if you want to lower your HR, just ride easier, or at least more consistently. Don't sprint up hills and out of corners, just focus on your breathing and keeping a steady effort. Ultimately, if you want a lower HR for a given speed/power, you just need to improve your fitness.