Because there is the limit of 200 segments at 1 time, when area has more, you will see only some of them. Zoom in, to see other segments.
You can check my FAQ - Segments & HeatMap.
And yes, they should make it better like other maps, which groups many locations as 1 icon, when looking on a large area - then would be 1 icon showing "there are 50 items here", and they would separate after zoom in.
