Why didn't the dermatologist remove "everything" off the mole in the first place?

I went to the dermatologist to get checked for any moles that were or could become carcinogenic. He removed a mole and sent it for analysis. After a week, I got a call saying that it was not carcinogenic but that it had the potential. Therefore, they asked me to schedule a new appointment to make sure nothing of that mole was left over. Could any body agree or disagree with the fact that this could simply be done in the first visit?