Diamonds are not created equal
I recently had a client come in with a quote from a competitor. The quote was for a 1.50 carat, F/VS1, Ideal Cut round diamond, GIA certified. I found a diamond with the same parameters. My quote was higher than the one my client received from another broker (which I pointed out was an unheard of price for that quality of diamond). I asked if he had a copy of the certificate. He did happen to have one and showed it to me. Well, the answer jumped out at me immediately. The polish and symmetry were graded Very Good, where my diamond was graded Excellent/Excellent. I explained to my client that diamonds with excellent finish will cost more due to superior craftsmanship and expertise.
Ideal Cut diamonds fall under strict proportional parameters. An Ideal Cut round brilliant requires a finish grade of Very Good to Excellent for both polish and symmetry. Very Good polish means there are minor polish imperfections such as polish lines on a couple of facets. Very Good symmetry could mean there are minor misalignments between facets.
Though a Very Good grade is, well, very good, it is not excellent. It takes a diamond cutter longer to achieve a perfect polish on every facet and superior skills to cut perfect facets that are all aligned.
Bottom line...you get what you pay for.