Lithium (EN $=1.0$ ) is less electronegative than $ Mg ( EN =1.2), $ therefore, organolithium compounds are more nucleophilic than Grignard reagents. As a result, organolithium compounds add more reactive $CO _{2}$ as well as less reactive resonance stabilised lithium salt of carboxylic acid thus formed to produce ketones.
Grignard reagents on the other hand, being less nucleophilic add only to the more reactive $CO _{2}$.
