Simple - think about the range and angles
The radar waves are not parallel to the ground they are at a slight angle typically a couple of degrees.
The further away from the radar the greater the distance between the ground and the lowest radar wave.
The closer you are the smaller the angle.
Miami has several radars that feed into their system - I would imagine on the 2nd leg you were simply further away.
Radar is affected by many factors but range is a basic one.
This is the best image I can find - it's for weather radar but still illustrates the point
https://encrypted-tbn1.gstatic.com/i...WtTaiuYfrBKkTw
The other alternative is that the controller didn't have the capacity to control you as VFR - remember in the USA flight following is workload permitting for controllers.