I don't bet halftimes, so I don't follow this stuff very closely, but I do know that most sports do have a "comeback tendency", and that this is built into the line.
So in the first example, Dallas is -3.5 for the game. On the average, I'd guess that teams that are 3.5 point favorites are favored by about 1.5 or 2 points in the second half, but it varies depending on the score, so in any particular game the 2H line might be far greater or less than -2. Since Dallas was trailing and there is a tendency for trailing teams to come back, they should be favored by more than 1.5-2 points in this case. (And obviously if they had been ahead big, they would be favored by less than 1.5-2 points and might even be the underdog in the 2H line.)
The other thing to remember is that in this case, -2.5, -3 and -3.5 are all the same line, because the second half can't land on 3. (I'm assuming 2H lines include overtime, correct me if I'm wrong.) So you can think of -4 as being really only one point away from -2. Because Dallas should probably be favored by more than 2, so -4 looks pretty reasonable to me.