Aureus

It's to do with the Setting up Units and the Reserves rules sections in the Core Rulebook, they both say that in certain situations it is possible to "set up" units in locations other than the battlefield and then later deploy them on the battlefield. So they still count as being set up, just not on the battlefield yet. And then due to how Magmic Tunnelling is worded, the two units count as two separate units for drops because you set up the Runesmiter in Reserve first and then after that one other unit can join him in Reserve instead of being set up on the table. I'd love for them not to count as drops cause then my Fyreslayers would be a three drop instead of the seven they currently are.

How are you getting down to two drops? You have six at the minimum, the Runesmiters and Vulkites all count as a drop each even if you tunnel them, unless I've missed something that's changed recently that would really benefit me.