different face pairs of LFW has high similarity, why?

environment:
(1) retinaface(R50) for face detection and alignment
(2) arcface(r50) for face feature extraction
(3) cosine similarity

I test LFW face pairs using arcface, the same face pairs has higher cosine similarity, around 0.8~0.9, but different face pairs also has high cosine similarity, around 0.6, i think it is abnormal, but i can not make out the issue. can you share me with some idea?