at 6 weeks they want to see a h/b of over 100 since yours was right on the mark they probably just want to make sure... good luck
An ultrasound at 6 weeks is still rather early, so try not to worry about the lower heartbeat. That's probably why the doctor wants to see another, to make sure the heartbeat picks up as baby gets older.
I am guessing it is because a heartrate of 100 is low for a fetus. Generally speaking the rate is usually around 140-160 beats/minute. Let them take a look at your fetus again to make sure that the fetus is having an appropriate heartrate and that he or she is developing normally.