I just created a new Annex by doing the following:

 1. git init
 2. git annex init
 3. git annex add .
 4. git commit -m "Added files"
 5. git annex status

I see the following:

    local annex keys: 224
    local annex size: 41 gigabytes
    known annex keys: 235
    known annex size: 49 gigabytes
    bloom filter size: 16 mebibytes (0% full)
    backend usage: 
            SHA256: 459

Why is there an 8 gigabyte difference here?  What/where are those files?  What is a bloom filter?