When a car is bought by a dealer (part exchange or bought at auction etc), the car is "put into trade" but they are not regarded as being a new keeper.
Did the dealer knowingly and deliberately mislead people by stating the wrong number of keepers or did the person putting the details on the advert just pull the info from the V5 blindly and without thinking? Who knows.
TBH, anyone that is really bothered about these things when buying the car should ask to see the V5 and MOT or even just confirm with the dealer by asking the question before buying the car rather than trying to solely blame the dealer.
People generally dont like car sales people and they think they are always trying to shaft them so, with this opinion, why people blindly go in and dont ask relevant questions is beyond me