Less free space after securely erasing free space?
That's it. Do you have external SSD and FileVault enabled? Having SSD drive, Secure Erase and Erasing Free Space commands aren't needed for SSD drives 'cause a regular erase command makes it difficult to recover data from an SSD.

Also, if you don’t include freespace word, diskutil will start securely erasing the entire disk, instead of just the free space! 1 means single-pass method run of random numbers. 

diskutil secureErase freespace 1 /Volumes/SSD


Be careful with it. Securely erased means there’s no chance you’ll be able to recover the data.

Read this useful AD post: Why is a secure erase 'not necessary' for SSD ?