Both forward velocity and rotational velocity start to slow down as soon as the bullet leaves the muzzle, it's just that the forward velocity decelerates at a much higher rate than the rotational velocity, especially once the bullet impacts the animal or media.
We're also adding some confusion to the mix by interchanging the term RPM (revolutions per minute, which starts to decrease slowly as soon as the bullet leaves the muzzle), and revolutions per inch, which starts to increase as soon as the bullet leaves the muzzle. Even if the bullet's rate of rotation stayed completely constant as the bullet penetrated, we would see the RPI increase as the bullet moves forward, since the forward motion is decelerating, meaning that the bullet continues to spin at the same rate, but it isn't travelling forward as fast.
