According to the ObjectID specification, 5 of the bytes are meant to be per-process and random. The BSON library currently has a bug however where the first and last bytes of this random section are always 0.

repro:

let oid = bson::oid::ObjectId::new();
println!("{:?}", oid.bytes());       

example output:

[96, 254, 225, 198, 0, 42, 28, 13, 0, 2, 174, 186]