While they do look very similar in the videos, it's kind of an immediate judgement call - the following is attempting to rationalize the line judge in a fraction of a second differentiating "Okay and Oshit". First was deemed a jump that could be defended or called if needed, Mercilus is 1 yard in at the LT. Second was 'unabated to the quarterback' if I remember correctly which was called quickly with how far the defender was before the line moved, 2 yards this time and outside of the tackle. Both stopping is what makes the plays look similar.
I think the call vs Watson was premature and should've been allowed to continue for at least another beat, but I can see why they were ruled that way - because quarterbacks are special flowers. Had Mercilus continued forward the play most likely would've been blown dead as well.