Purely the positional adjustment.
It's difficult to compare defensive value. If I've got a SS who is 5 runs above average at SS, we don't think he's got the same defensive value as a 1B who is 5 runs above average at 1B. SS is the harder position, so the defense is more valuable there.
So about 20 years ago, Tom Tango came up with the notion of the positional adjustment, to give an idea of how valuable each position is. A bunch of math went into it. And people continue to debate just how accurate that is.
But generally speaking, Catchers get the biggest innate dWAR bonus. SS also get a substantial bump. 2B/3B/CF get a little bump. LF/RF take a substantial hit to their defensive value. And 1B/DH get a bunch of value deducted.
Those are just the starting values. The remainder of the dWAR calculation comes from comparison to their peers at the position, which isn't really applicable for full-time DHs.