Understanding 'availability' data point

Hello -

We have noticed that the data percentage of availability does not equate to what was planned. We have some resources assigned to 100% of the available time but we still see that the resource has some availability noted in the schedule view. We have ensured number of working days/hours is correct. We do allocate resources in hours, not percentages.

Can anyone help understand why this would be and or how to change it?