The verse says there are 'certain things' hard to be understood (not impossible), which the Unlearned and Unstable wrest (twist and turn to meet their view of what they want scripture to say) as they do the rest of the scripture...
I take it you think Scripture is easy to understand, it says otherwise:
Scripture that was written for us is not difficult to understand...Believing it is where most people crash and burn...
Therefore "mysterious".