Because lets say the expressed spider gene causes a gap in the allele that is responsible for proper motor function. It is possible that since another mutation has the complete allele that it could fill the missing gap and in turn restore proper motor function. It's 100 percent possible that the gene that causes the spider mutation also causes other physiological like vertigo that can be corrected by additional gene's..It's done in other life forms all the time; I.E. Cows, vegetables, horses, people gene replacement or supplement therapy is a possible solution's to debilitating genetic disorders.