Delete Version Chains

I have my simple scheme set to do a Full backup and then daily differentials, with a new full after 7 backups. I set the clean-up to delete version chains older than 10 days. The backing-up is working fine; I got a full, then 7 differentials. Then a new full backup, and now so far four differentials after the new full backup - but it isn't deleting the previous chain??? If I have it set to delete after 10 days, is that 10 days after each backup and they delete one by one, or 10 days after the last backup in that chain?
Thanks.