Chapter 8, Problem 8.64P

Explain why the reaction of $2-\text{bromopropane}$ with ${\text{NaOCOCH}}_{\text{3}}$ gives ${\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}{\text{CHOCOCH}}_{\text{3}}$ exclusively as product, but the reaction of $2-\text{bromopropane}$ with ${\text{NaOCH}}_2{\text{CH}}_{\text{3}}$ gives a mixture of ${\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}{\text{CHOCH}}_{\text{2}}{\text{CH}}_{\text{3}}\text{(20\%)}$ and ${\text{CH}}_{\text{3}}\text{CH}={\text{CH}}_{\text{2}}\text{(80\%)}$.