Why invert the pitch Angle in the tag, when using biwi data

[zn1966]

In RankPose/src/dataset/biwi.py

with open(filename, 'r') as f: # data/biwi_dataset_list.txt
for i, line in enumerate(f.readlines()):
ls = line.strip().split(' ')
bbox = [float(ls[i]) for i in range(1,5)]
pose = [float(ls[i]) for i in range(5,8)]
pose[1] = - pose[1]
if True and (abs(pose[0])>99 or abs(pose[1])>99 or abs(pose[2])>99):
continue

I mean why need to invert pose[1]?