oh, maybe when he's talking to tarkin he says tremor. he could have just used tremor as in a change, i mean, besides him and the emperor vader wouldnt have felt any fluctuation or any thing in the force, then someone else that can use the force shows up and it causes a change, its been a while but he recognizes it as obi-wans presense and doesnt notice that 'there is another' so to speak.
anywway, i dunno. what were we talking about again that got us here
-Darth Simon