As I understand it, SMART is only to detect hardware problems with your disk. This is not a hardware problem, this is data corruption in your directory. Meaning your hard disk can physically be working fine, while at the same time still losing all your data in a worst case scenario.
The directory is basically a map that says where each file on your disk is located, just like a map would say the toilet is in the bathroom. When your directory is corrupt it will say that files are in a place on your disk where they are not actually located, like a map saying the toilet is in the bedroom. If the damage is serious the directory may say that an area on your disk is free when it is actually occupied by your family photos, then when another program needs to write data it will overwrite the sectors containing your photos and then goodbye picture of baby's first steps.
Serious directory corruption is very rare, but if disk utility can't fix the problem then it's quite a bad sign (permissions that can't be fixed are often fine, I'm talking about volumes that cannot be fixed). I don't know what your computer habits are, but there are several activities that can invite this sort of trouble. For instance, if you use pirated software, if you run software that is still beta or from a small company of unknown reputation, or if you turn off your computer without doing a shutdown first, or things like that.
If I were you I'd solve this issue before it becomes an actual problem and you lose data.
