Discrepancy Between Physical Disk and Only Partition

After much effort, including a fresh reinstallation of Mac OS, I was able to get rid of a troublesome Bootcamp partition (which was really only troublesome due to my own incompetence). Though I am down to only one partition, there is still a discrepancy between the physical disk (250.43 GB) and the "Mac HD" partition (209.92 GB). Any suggestions as to how I might reclaim this unusable, mysteriously disappeared space? Thanks in advance for your help!

Mid-2012 MBP Retina

OS X El Capitan (10.11)