my s3 mini android version says 4.1.3, but i didnt even know this version existed...
That's one I'm unfamiliar with as well. 4.1.2 is the last version of API Level 16 that I'm aware of. A month after it was released, 4.2 was released introducing API Level 17.
does it mean my phone is a fake one? :/
I'm not sure. If that's a concern, I'd definitely recommend having that investigated. If it is a legitimate device, I think the S3 Mini should be upgradable to 4.2. It's not unheard of for an OEM to have a version number that's unique to them, but I wasn't able to find out much of anything about 4.1.3 via Google either.