Hmmm, some examples (Not knowing the crown and pavilion angles, I can only ascertain that at least some of these diamonds' characteristics fall within the parameters of a 1A cut): All GIA certed, H, VS1, 2.00-2.05ct, table 53-58%, depth 58.7-62.3%, very good or excellent polish and symmetry, girdle: medium, thin - medium, or medium - slightly thick.
However, I think you are on the right track by making cut a priority. And please do not make your decision regarding the cut based on table, depth, polish, and symmetry alone. Ask for a Sarin report and plug the #s into the HCA, at the very least. Good luck!
To answer your question, the reason for the variances in price is that you are NOT comparing apples to apples. When you buy a superbcert, you can view the diamond's brilliance scope and know that you are getting a highly regarded brand. If a diamond is suspiciously discounted below other diamonds of the same specs, it probably has a less desirable cut. Contact the seller and ask for the results of a Sarin report or consider AGS certed diamonds, which already come with the cut information.