Most likely the task itself has the Maximum Units value set to a number, which is smaller than the resource capacity. You can check that by clicking the task in the Gantt chart and checking the Max Unit value in the Task Details Popup.

This value defines the default maximum number of resource units that can be allocated for a task. It can be set for each individual task in the Task Details panel (see Gantt Chart Elements) and the default value can be set in the Gantt configuration.