Family sharing at find my iphone - tracking of the person works, but the device does not

My son has an iphone and has set up family sharing to see where he is.

When I go into "devices" in find my iphone it shows last updated 2 days ago. However if i go into "persons" he shows as active at school a few minutes ago. The phone is the only apple device he has, so its the phone that also identify where he is as a person. Why does the "device" function not update at the same time?