Why the maximum error is 0.5 when using PER?

[hipoglucido]

https://github.com/cmusjtuliuyuan/RainBow/blob/252f7ef80823b0d52b2992e05b38c9cbd7aed72b/replayMemory.py#L70 When a new experience is going to be added in the PER memory it should be registered with the maximum priority hence the the maximum error. Why the maximum error is 0.5 when using PER? If I understood the paper it is said that maximal priority should be taken as the current maximum priority value that exists in the memory.