Why is only one ankle slightly swollen?

Only one ankle of the family member is swollen.
When I measured the ankle circumference, the swollen side was about 1cm larger.
Is there a special reason for swelling only in one ankle?
There is no swelling at all in the calves or feet.
They say there is no numbness or pain.
Do you experience swelling in only one ankle? Is there a specific reason for it?