Teleporting nav mesh agent in stead of moving

I've already resolved my problem but I don't understand why it is now working and wasn't before...

I have this piece of code inside a function to set the destination of my agent. It is called every time the agent comes within 2f of its destination. It was working fine like this:

     //wanderingRange is just how far the agent moves from his original spawnposition

     int randomX = Random.Range((int)-wanderingRange, (int)wanderingRange);
     int randomZ = Random.Range((int)-wanderingRange, (int)wanderingRange);

     Vector3 newDestination = new Vector3(randomX,
                                          transform.position.y,
                                          randomZ);

     agent.SetDestination(spawnPosition + newDestination);

     currentDestination.position.Set(newDestination.x,
                                     newDestination.y,
                                     newDestination.z);

Then I changed it to this, thinking that it was shorter:

 Vector3 newDestination = new Vector3(
                         Random.Range((int)-wanderingRange, (int)wanderingRange),
                         transform.position.y,
                         Random.Range((int)-wanderingRange, (int)wanderingRange));

 currentDestination.position = spawnPosition + newDestination;

 agent.SetDestination(currentDestination.position);

Now in this second piece of code, the agent all of a sudden started teleporting in stead of moving and basically was in a completely different location each frame and calculated a new destination every frame. I have no clue how this is possible but it is the only change I made in the code and after reverting it back to the 1st snippet, it started working normally again.

So my real question here is; how is one piece of code working and the other one isn't?