cause you're the only person left that uses your phone as a phone :P.
Mine never seems to alert me if it's on vibrate except for texts since I swapped to GOSMS pro. As for calls.. in silent it doesn't vibrate for a phone call even.. or if it does I have not felt it once in the time I've had the phone.. and it's a strong vibrate at that. With my volume all the way up it works fine. Since I usually watch the LED when in silent and need the phone to not vibrate anyway most of the time, that works out in my favor.. maybe it's similiar for others in that respect?