Time Machine Backup over twice the size of my rMBP drive!?!?

I just purchased a 3 TB Airport Time Capsule to run time machine on. I used to send the backups to my drobo, but was running low on space there and wanted to upgrade to 802.11 AC as I have a new rMBP and this would kill two birds with one stone.

Got the Airport up and running and am in the updating process. My rMBP has the 1 TB drive of which 188 gigs are free. My Time Machine backup is estimated at 2.12 TB? WTF? not sure where the other 1.3 TB are coming from? I have attached drives to the rMBP but in the Time Machine preferences I made sure they weren't included.

What gives? I was hoping to use this drive to store extra data (GoPro files etc) but at this rate, it will be full in a week!!!

I guess I will wait and see what it finally comes up with, but at this point it has been going since last night and still has 20 hours to go.

My rMBP HD has 812 GB used. The Time Machine has backed up 820 GB of 2.21 TB. I just can't understand where the additional 1.3 TB comes from.

I know that the size is an estimate, but I don't really understand how the backup works. Is it replicating files? If I only have 1 TB to back up where does it find the additional data? What is it?

so after further scrutiny I discovered that time machine was backing up one of my attached drives. I have a drobo that I have partitioned and upon examination into the excluded drives I saw that the main Drobo was excluded but the ancillary wasn't. There was 1 TB on there that was being backed up. I am in the process of erasing the time capsule and starting over.