As I understand it, there's a clear legal difference. Sotheby's make representations about artworks in their catalogues which they are liable for if they prove to be false. Their related company RM Sotheby's explicitly disclaims any responsibility for cars not being as described. The following is taken from the Bidders' terms & conditions for their Monterrey sale:
5. All Sales are “As Is” and “Where Is.” The Bidder is responsible for inspections and verification of condition, authenticity, and completeness of any lot purchased. No warranties or representations of any type whatsoever are made by RM. Statements printed in catalogues, brochures, signs, and window cards, as well as verbal statements made by auctioneers or auction staff, are representations made by the consignor, and RM has no obligation to verify or authenticate any such claims or representations. Any announcements made at the time of sale supersede any earlier printed information. Except as herein provided, all lots are sold as is, where is, with no representations or warranties, expressed or implied. THE CONSIGNOR AND RM DISCLAIM ALL WARRANTIES, EXPRESSED OR IMPLIED, AS TO CONDITION, ORIGINALITY, OR AUTHENTICITY; ORIGIN OR PROVENANCE; PREVIOUS USE OR OWNERSHIP; MANUFACTURING OR RESTORATION PROCESSES; YEAR OR AGE; SERIAL NUMBER, MAKE, OR MODEL; OPTIONS AND TOOLS; ENGINE HOURS; AND MILEAGE OF ANY LOT OR COMPONENT OF ANY LOT, AND THEY SPECIFICALLY DISCLAIM ANY WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE.