Why bolt.Bucket.FillPercent is set to 0.9?

[cenkalti]

I found that the concept is introduced with d883d5a but don't see a reason why it is set to 0.9 instead of 1.0. Doesn't it make sense to set 1.0 for better page utilization?

I see that boltdb compaction method (which is not used by etcd) sets it to 1.0:
https://github.com/etcd-io/bbolt/blob/da2f2a53f6e2f25b215b79db2cd417488ef8e955/compact.go#L61

[ahrtr]

Please read #15470 (comment)