Why the connectivity paths only have non-zeros density/intensity at the last few entries

I tried to extract the paths of mouse brain connectivity of the injection experiments. Taking one path lines[60]['path'] as an example, why only the third to last and second to last entries have non-zero values for density and intensity?

It’s not just the case for lines[60]. I also tried other lines[i], and I noted that generally only the second to last (or third to last as well) entries are non-zeros, and all other entries are zeros. Why would that happen?

injection_data_set_id = 156394513

x = 4400
y = 3300
z = 5600

url = LINES_FMT % (x, y, z, injection_data_set_id)

connection = urllib.request.urlopen(url)
response_text = connection.read()
response = json.loads(response_text)

lines = response['msg']