why would this be considered a mutual mistake?

On 1 November 2016, Ben took Ming for a tour of the partially constructed Boxy Shopping Centre. He showed Ming a large store on the southern end of the centre which had been leased by Bullworths, then another spacious store at the northern end. Ming was under the impression that she would be leasing the southern store as this was closer to the carpark and seemed to be a better location. Ben did not tell Ming that the southern store was available to be leased by her. Ben thought that Ming was intent on leasing the northern store in Boxy. Ming made positive comments about both the stores she was shown.