How can a mole biopsy be inconclusive?

[Confusedpatient2017]

Five years ago I went to my PCP for an unusual mole. He did a punch biopsy and the results were normal.

A few weeks ago I went to a dermatologist for the first time for a case of eczema. While there the doctor performed a routine skin check and noticed this mole.  I told her it had been biopsied before. She looked up the results and agreed they were normal. However, she said it was a really unusual mole and the only way to biopsy the entire mole is to remove the entire mole - so the mole came out. Yesterday I went in to the doctor to have the stitches taken out. While there the doctor told me they "think it might be melanoma but they aren't certain so they have requested the old biopsy sample to compare the tissues."  It will take a week or so to get the results.  I was stunned at the news so I didn't have the capacity at the time to think of questions.

My questions now are how do they not know? Isn't it either cancer or not?  What are they looking for by pulling the old sample? And if it is a hard to diagnose case should I seek a second opinion no matter what they tell me the results are?

Thank you!