Default [Q] EFS partition restoration

I looked at other threads regarding the EFS partition but none that really answer my question. I hope it's not a stupid question...

So basically, I was trying out a new ROM and made a backup using TWRP. I inadvertently backed up the data partition separately, but I figured that didn't matter. I then backed up all other partitions (EFS and Modem included) in a separate file. I didn't like the new ROM I tried so I went to restore my backups. When restoring the "everything sans data" backup, I kept getting an md5 error, even though I didn't create an md5 upon backup nor did I ask it to look for an md5 on restoration. I decided to try and restore everything except the EFS, and it worked. Phone is up and running and it doesn't seem like there are any issues. Can someone please explain this to me? I'm assuming the existing EFS partition was never changed, so restoring everything but the EFS should still result in a complete system, but can anyone suggest a reason why it failed when trying to restore EFS with everything else? Thanks!