Sales normally dwarf purchases here, and that is the baseline rather than a warning. Executives are paid in stock and sell to diversify or to cover the tax due when awards vest, so a large sell total across a sector usually reflects a vesting calendar. Buying is the informative side, because an insider purchasing on the open market is spending their own money on a company they already understand.
Companies are ranked by distinct insiders filing rather than by dollar value, which surfaces the cluster case: several people inside the same business acting in the same window. Where a company reports only an SEC SIC code, it is mapped into these sectors through a standard crosswalk, so the grouping reflects what a business does rather than how it happens to file.