The asteroid's orbit is not changing, so the likelihood of it hitting Earth is not changing. What is changing is our knowledge of its orbit, which improves as

The asteroid's orbit is not changing, so the likelihood of it hitting Earth is not changing. What is changing is our knowledge of its orbit, which improves as new observations are added over time. Doing that shrinks the volume spanned by all trajectories compatible with the data. While the uncertainty is still large compared to the volume of the target (Earth), this shrinkage removes empty space, so it increases the odds. If the actual orbit does not intersect Earth, the odds will start going down once further shrinkage removes trajectories which do.